Add methods related to graph homeomorphisms

[dcoudert]

This PR:

• adds a new module to gather methods related to graph homeomorphisms, homomorphisms, etc.
• add methods for checking if two graphs are homeomorphic
• move method has_homomorphism_to from graph.py to this new module and extend it to make it work for digraphs

Note that homeomorphisms and homomorphisms are very different notions in graph theory.

📝 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]

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