Path to featomic release

[jwa7]

Here we outline the steps needed to get to a rascaline release.

• Build steps, outlined in Port over build system improvements from metatensor #281
• Renaming metadata to match new conventions in metatensor Rename all metadata according to the new convention #285
• Find a product name, free on PyPI, GitHub, crate.io, no problematic related Google results

[Luthaf]

• Separate cell and strain gradients and fix cell gradients for small cell/large cutoff calculations

[jwa7]

• New hypers structure for base calculators

[Luthaf]

I don't think that anything related to the CG routines is related to the release. This issue is about stuff we can NOT do a release without, but CG can always be added/modified after an initial release (that's what incompatible version numbers are for). If you want to keep track of everything CG related somewhere, could you open a separate issue?

[jwa7]

Ok that makes sense, I'll move those