sagemathgh-36924: add interface to nauty's genktreeg
    
Nauty has a generator of k-trees. We add an interface to this generator.
This is a complement of sagemath#36587.

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### ⌛ Dependencies

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URL: sagemath#36924
Reported by: David Coudert
Reviewer(s): Matthias Köppe

sagemathgh-36924: add interface to nauty's genktreeg
    
Nauty has a generator of k-trees. We add an interface to this generator.
This is a complement of sagemath#36587.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

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

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- sagemath#12345: short description why this is a dependency
- sagemath#34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
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URL: sagemath#36924
Reported by: David Coudert
Reviewer(s): Matthias Köppe

sagemathgh-36587: Created functions to generate random k-tree and partial k-tree graphs
    
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes sagemath#1234" use "Introduce new method to
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<!-- Describe your changes here in detail -->
Implemented graph generators which produce a random k-tree and a random
partial k-tree. The k-tree algorithm first generates a complete graph,
before adding vertices one by one, creating a clique utilising random
existing vertices and the new vertex. Partial k-trees are generated by
producing a k-tree and then randomly removing edges from it.

<!-- Why is this change required? What problem does it solve? -->
Previously SageMath lacked a way to generat randomised graphs which have
a fixed treewidth value. This provides a methodology to do so .
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes sagemath#12345". -->
Addresses sagemath#36586
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appropriately. -->

### 📝 Checklist

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URL: sagemath#36587
Reported by: Damian Basso
Reviewer(s): Damian Basso, David Coudert

sagemathgh-36587: Created functions to generate random k-tree and partial k-tree graphs
    
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes sagemath#1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->
Implemented graph generators which produce a random k-tree and a random
partial k-tree. The k-tree algorithm first generates a complete graph,
before adding vertices one by one, creating a clique utilising random
existing vertices and the new vertex. Partial k-trees are generated by
producing a k-tree and then randomly removing edges from it.

<!-- Why is this change required? What problem does it solve? -->
Previously SageMath lacked a way to generat randomised graphs which have
a fixed treewidth value. This provides a methodology to do so .
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes sagemath#12345". -->
Addresses sagemath#36586
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
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appropriately -->
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here to help! -->
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- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.


<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: sagemath#36587
Reported by: Damian Basso
Reviewer(s): Damian Basso, David Coudert