Add GAP model

[frostedoyster]

Reopening #76

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📚 Documentation preview 📚: https://metatensor-models--223.org.readthedocs.build/en/223/

[frostedoyster]

I had to soft reset the branch due to the history rewrite. Looking for help on how to make @DavideTisi and @agoscinski co-authors of this PR

[Luthaf]

Looking for help on how to make @DavideTisi and @agoscinski co-authors of this PR

When committing you can add them by including Co-Authored-By: Some One <some.one@example.com> in the commit message. You can do another soft-reset to add this to the main commit, or we can do this when squash-merging the branch

[DavideTisi]

so now that the "main" branch is in main what is missing to merge this PR?

[PicoCentauri]

Very good work @DavideTisi @agoscinski and @frostedoyster !

[PicoCentauri]

MERGE!!!11!!!

If I understood @frostedoyster correctly we now want to replace the example with the GAP model and train the model directly in CI?

Also do we want to use GAP for the tests of the main library? Should also speed up the tests compared to the BPNN.