sagemathgh-39923: add parameter immutable to GraphQuery `src/sage/graphs/graph_database.py`
    
Following discussions in sagemath#39177, we add the option to return immutable
graphs when querying the graph database.

We also exchange some code between methods `query_iterator` and
`get_graphs_list` to avoid the creation of a list when we only want to
iterate over the answers.

### 📝 Checklist

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- [x] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: sagemath#39923
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

sagemathgh-39923: add parameter immutable to GraphQuery `src/sage/graphs/graph_database.py`
    
Following discussions in sagemath#39177, we add the option to return immutable
graphs when querying the graph database.

We also exchange some code between methods `query_iterator` and
`get_graphs_list` to avoid the creation of a list when we only want to
iterate over the answers.

### 📝 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.
- [x] I have created tests covering the changes.
- [x] 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#39923
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

sagemathgh-39923: add parameter immutable to GraphQuery `src/sage/graphs/graph_database.py`
    
Following discussions in sagemath#39177, we add the option to return immutable
graphs when querying the graph database.

We also exchange some code between methods `query_iterator` and
`get_graphs_list` to avoid the creation of a list when we only want to
iterate over the answers.

### 📝 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.
- [x] I have created tests covering the changes.
- [x] 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#39923
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

sagemathgh-39923: add parameter immutable to GraphQuery `src/sage/graphs/graph_database.py`
    
Following discussions in sagemath#39177, we add the option to return immutable
graphs when querying the graph database.

We also exchange some code between methods `query_iterator` and
`get_graphs_list` to avoid the creation of a list when we only want to
iterate over the answers.

### 📝 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.
- [x] I have created tests covering the changes.
- [x] 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#39923
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

sagemathgh-39923: add parameter immutable to GraphQuery `src/sage/graphs/graph_database.py`
    
Following discussions in sagemath#39177, we add the option to return immutable
graphs when querying the graph database.

We also exchange some code between methods `query_iterator` and
`get_graphs_list` to avoid the creation of a list when we only want to
iterate over the answers.

### 📝 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.
- [x] I have created tests covering the changes.
- [x] 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#39923
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton