Add metatensor models

[orionarcher]

Recreated from #124

Hi everyone and thanks for the great work on the library. Also, let us know if you have feedback on https://github.com/Luthaf/vesin!

Summary

• This PR adds an interface to the metatensor family of models (see https://docs.metatensor.org/latest/atomistic/index.html)

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[orionarcher]

@frostedoyster, sorry for all the confusion on this PR! Reopened this, is it good to merge?

[frostedoyster]

Yes, I think it's good! It seems that it might be a good idea to increase the tolerance on graphpes a bit

[orionarcher]

Thank you! Will merge now (again)

It seems that it might be a good idea to increase the tolerance on graphpes a bit

Yep! I have that in #143