Rascaline -> featomic renaming

[frostedoyster]

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📚 Documentation preview 📚: https://metatrain--392.org.readthedocs.build/en/392/

[frostedoyster]

@Luthaf help desperately needed

[Luthaf]

Can this wait a bit and still pull rascaline from a previous commit? I still need to move the repo to the metatensor organization.

[Luthaf]

The issue in CI seems to be

Warning: The model at 'model_no_zbl.pt' was exported with extension 'featomic_torch' loaded (from 'featomic/torch/lib/libfeatomic_torch.so'), but it does not seem to be currently loaded; proceed at your own risk. (function check_atomistic_model)

We will likely need to update some of the code in metatensor-torch first as well.

[Luthaf]

As part of this, we should also try to use the CG code from featomic, as mentioned here: #444 (comment)