add parameter immutable to graph generators in smallgraphs.py (part 1)

[dcoudert]

Following discussions in #39177, we add the option to return immutable graphs to some generators in src/sage/graphs/generators/smallgraphs.py.

• HarborthGraph, Cell600, Cell120, SuzukiGraph, HallJankoGraph, BidiakisCube, BlanusaFirstSnarkGraph, BlanusaSecondSnarkGraph, BrinkmannGraph, BrouwerHaemersGraph, BuckyBall, GossetGraph, DoubleStarSnark

📝 Checklist

• The title is concise and informative.
• 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 and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 65d6b79; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[videlec]

This new option is tested nowhere... is that intended?

[dcoudert]

I added a few doctests.

[dcoudert]

I can do the same for the other PRs. I did not do it by default as in most of the cases it's rather straightforward and file small graphs.py contains around 100 methods...

[videlec]

Thanks.

For purpose of documentation I think it is fine to only have a bunch of them. However, for testing purposes it would be nice to have a test (via pytest?) of the form

for my_constructor, args, kwds in graph_constructors:
    G = my_constructor(*args, **kwds, immutable=False)
    # test mutable
    G = my_constructor(*args, **kwds, immutable=True)
    # test immutable

[dcoudert]

I keep this idea for a future PR, after the ongoing ones will be merged. There is still a lot of work to do for small graphs as some constructors depend on constructors from other modules that have not been touched yet (for instance to reduce potential conflicts).

[videlec]

Sounds good to me!

[dcoudert]

Thanks.