Add OPTIMADE adapter

[ml-evs]

Summary

This PR adds a converter between the pymatgen.core.Structure object and an OPTIMADE structure resource, migrated primarily out of https://github.com/Materials-Consortia/optimade-python-tools/blob/master/optimade/adapters/structures/pymatgen.py with the aim to remove it from our library.

Todos

• Finish off tests
• See if it can be used internally in the pymatgen.ext.optimade client (future work)

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

[shyuep]

I would put this in pymatgen.ext rather than pymatgen.io. The ext are external interfaces to things like API resources. E.g., matproj is in pymatgen.ext. Thanks.

[mkhorton]

Given this is purely for conversion to/from the OPTIMADE schema, io is appropriate (but yes, any API-calling code lives in ext).

Thanks for the PR @ml-evs !

[ml-evs]

Apologies, I made this during the OPTIMADE workshop and your responses got lost in the notifications -- happy for this to go wherever but agree with Matt that io seems more appropriate to me (given that more tools are adding "offline" OPTIMADE support).

I'll get around to adding a couple more tests to this, but I think the code itself is ready for review.

[ml-evs]

Ok, slightly concerned that rebasing this PR has lead to 395 files being changed...

[janosh]

Ok, slightly concerned that rebasing this PR has lead to 395 files being changed...

that's from the recent 9100860 which didn't ensure pymatgen is recognized as a first-party package

[ml-evs]

Ok, rebased this more intelligently this time, have beefed up the test a little and think its ready to go.