Support parsing unit cells

[padix-key]

This PR adds parsing an AtomArray/AtomArrayStack representing an entire unit cell from PDBx files.

• Added space_group_transforms to get transformations for a given space group
• Added pdbx.get_unit_cell() to get the unit cell AtomArray/AtomArrayStack from a PDBx file
• Renamed pdb.get_symmetry_mates() to pdb.get_unit_cell() (kept the old version, but deprecated it)

Resolves #660.

[codspeed-hq]

CodSpeed Performance Report

Merging #778 will not alter performance

_{Comparing padix-key:unit-cell (35a6af0) with main (5465b75)}

Summary

✅ 59 untouched benchmarks