sagemathgh-37692: `matrix`, `Graph.incidence_matrix`, `LinearMatroid.representation`: Support constructing `Hom(CombinatorialFreeModule)` elements
    
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We use morphisms of `CombinatorialFreeModule`s (each of which has a
distinguished finite or enumerated basis indexed by arbitrary objects)
as matrices whose rows and columns are indexed by arbitrary objects
(`row_keys`, `column_keys`).

Example:
```
        sage: M = matrix([[1,2,3], [4,5,6]],
        ....:            column_keys=['a','b','c'], row_keys=['u','v']);
M
        Generic morphism:
          From: Free module generated by {'a', 'b', 'c'} over Integer
Ring
          To:   Free module generated by {'u', 'v'} over Integer Ring
```

Example application done here on the PR: The incidence matrix of a graph
or digraph. Returning it as a morphism instead of a matrix has the
benefit of keeping the vertices and edges with the result. This new
behavior is activated by special values for the existing parameters
`vertices` and `edges`.
```
            sage: D12 = posets.DivisorLattice(12).hasse_diagram()
            sage: phi_VE = D12.incidence_matrix(vertices=True,
edges=True); phi_VE
            Generic morphism:
              From: Free module generated by
                      {(1, 2), (1, 3), (2, 4), (2, 6), (3, 6), (4, 12),
(6, 12)}
                    over Integer Ring
              To:   Free module generated by {1, 2, 3, 4, 6, 12} over
Integer Ring
            sage: print(phi_VE._unicode_art_matrix())
                         (1, 2)  (1, 3)  (2, 4)  (2, 6)  (3, 6) (4, 12)
(6, 12)
                      1⎛     -1      -1       0       0       0       0
0⎞
                      2⎜      1       0      -1      -1       0       0
0⎟
                      3⎜      0       1       0       0      -1       0
0⎟
                      4⎜      0       0       1       0       0      -1
0⎟
                      6⎜      0       0       0       1       1       0
-1⎟
                     12⎝      0       0       0       0       0       1
1⎠
```



### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
- Depends on sagemath#37607
- Depends on sagemath#37514
- Depends on sagemath#37606
- Depends on sagemath#37646
    
URL: sagemath#37692
Reported by: Matthias Köppe
Reviewer(s): gmou3

sagemathgh-37940: Support morphisms in `Matroid` constructor
    
This follows the merging of sagemath#37692 and it enables the (re-)feeding of a
linear matroid's morphism representation into the `Matroid` constructor.

Example:
```
sage: M = matroids.catalog.Fano()
sage: A = M.representation(order=True); A
Generic morphism:
  From: Free module generated by {'a', 'b', 'c', 'd', 'e', 'f', 'g'}
over Finite Field of size 2
  To:   Free module generated by {0, 1, 2} over Finite Field of size 2
sage: Matroid(A)
Binary matroid of rank 3 on 7 elements, type (3, 0)
```
    
URL: sagemath#37940
Reported by: gmou3
Reviewer(s): gmou3, Matthias Köppe, Travis Scrimshaw

sagemathgh-37940: Support morphisms in `Matroid` constructor
    
This follows the merging of sagemath#37692 and it enables the (re-)feeding of a
linear matroid's morphism representation into the `Matroid` constructor.

Example:
```
sage: M = matroids.catalog.Fano()
sage: A = M.representation(order=True); A
Generic morphism:
  From: Free module generated by {'a', 'b', 'c', 'd', 'e', 'f', 'g'}
over Finite Field of size 2
  To:   Free module generated by {0, 1, 2} over Finite Field of size 2
sage: Matroid(A)
Binary matroid of rank 3 on 7 elements, type (3, 0)
```
    
URL: sagemath#37940
Reported by: gmou3
Reviewer(s): gmou3, Matthias Köppe, Travis Scrimshaw

sagemathgh-37955: `Graph.{[weighted_]adjacency_matrix,kirchhoff_matrix}`: Support constructing `End(CombinatorialFreeModule)` elements
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

This is a follow-up after
- sagemath#37692

... to cover a few more methods. The methods can now create
endomorphisms of free modules whose bases are indexed by the vertices.
To help with this, we make the `matrix` constructor a bit more flexible.

This is also preparation for making the spectral graph theory methods
ready for `CombinatorialFreeModule`:
- sagemath#37943

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37955
Reported by: Matthias Köppe
Reviewer(s): David Coudert, Matthias Köppe

sagemathgh-37940: Support morphisms in `Matroid` constructor
    
This follows the merging of sagemath#37692 and it enables the (re-)feeding of a
linear matroid's morphism representation into the `Matroid` constructor.

Example:
```
sage: M = matroids.catalog.Fano()
sage: A = M.representation(order=True); A
Generic morphism:
  From: Free module generated by {'a', 'b', 'c', 'd', 'e', 'f', 'g'}
over Finite Field of size 2
  To:   Free module generated by {0, 1, 2} over Finite Field of size 2
sage: Matroid(A)
Binary matroid of rank 3 on 7 elements, type (3, 0)
```
    
URL: sagemath#37940
Reported by: gmou3
Reviewer(s): gmou3, Matthias Köppe, Travis Scrimshaw

sagemathgh-37955: `Graph.{[weighted_]adjacency_matrix,kirchhoff_matrix}`: Support constructing `End(CombinatorialFreeModule)` elements
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

This is a follow-up after
- sagemath#37692

... to cover a few more methods. The methods can now create
endomorphisms of free modules whose bases are indexed by the vertices.
To help with this, we make the `matrix` constructor a bit more flexible.

This is also preparation for making the spectral graph theory methods
ready for `CombinatorialFreeModule`:
- sagemath#37943

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37955
Reported by: Matthias Köppe
Reviewer(s): David Coudert, Matthias Köppe

sagemathgh-37940: Support morphisms in `Matroid` constructor
    
This follows the merging of sagemath#37692 and it enables the (re-)feeding of a
linear matroid's morphism representation into the `Matroid` constructor.

Example:
```
sage: M = matroids.catalog.Fano()
sage: A = M.representation(order=True); A
Generic morphism:
  From: Free module generated by {'a', 'b', 'c', 'd', 'e', 'f', 'g'}
over Finite Field of size 2
  To:   Free module generated by {0, 1, 2} over Finite Field of size 2
sage: Matroid(A)
Binary matroid of rank 3 on 7 elements, type (3, 0)
```
    
URL: sagemath#37940
Reported by: gmou3
Reviewer(s): gmou3, Matthias Köppe, Travis Scrimshaw

sagemathgh-37955: `Graph.{[weighted_]adjacency_matrix,kirchhoff_matrix}`: Support constructing `End(CombinatorialFreeModule)` elements
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

This is a follow-up after
- sagemath#37692

... to cover a few more methods. The methods can now create
endomorphisms of free modules whose bases are indexed by the vertices.
To help with this, we make the `matrix` constructor a bit more flexible.

This is also preparation for making the spectral graph theory methods
ready for `CombinatorialFreeModule`:
- sagemath#37943

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37955
Reported by: Matthias Köppe
Reviewer(s): David Coudert, Matthias Köppe