Store symmetry group metadata in Model to avoid re-passing magnetic_moments

[niklasschmitz]

Problem

Several postprocessing functions (e.g. elastic_tensor) need to redetermine crystal symmetries for a modified lattice. Currently this requires passing magnetic_moments because the Model does not remember the symmetry group information needed to reproduce its symmetry operations.

This is awkward and error-prone, as discussed in #1172.

Proposed solution

Store symmetry group metadata (e.g. the spglib integer ID or pointgroup_symbol) in the Model so that symmetry-related queries can be made without re-supplying magnetic_moments. This should also be useful for band structures and other contexts where the same problem arises.

Relevant discussion: #1172 (comment)

Related: symmetry_from_database already supports integer group IDs, and Brillouin is compatible with this approach (unlike the string-based pointgroup_symbol).

[mfherbst]

the spglib integer ID

I think what you mean here is the ITA Spacegroup number, right ? https://spglib.readthedocs.io/en/latest/dataset.html#dataset-spg-get-dataset-spacegroup-type

[antoine-levitt]

Passing those magnetic moments around is a massive hack, it would be awesome to remove them!

[niklasschmitz]

Adding from #1172 (comment)

store tol_symmetry as part of the model next to the symmetry group information (i.e. the symmetry result can be bundled with its uncertainty)

[Technici4n]

It might also be good to store the symmetry tolerance alongside.