Re-allowing disordered Slabs

[Sanftperlig]

Re-allows the generation of Slabs from disordered species, like before #24.
This isn't a new feature, but the OUC from #24 used .species instead of .species_and_occu, accidentally disallowing the generation of disordered Slabs.

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

[Sanftperlig]

The formatting issue should be fixed now, it was my mistake for only running ruff on the non-test part, sorry.

[shyuep]

🤖 Automated PR review (generated by Claude on behalf of @shyuep)

Minimal, correct fix: switching oriented_unit_cell.species → .species_and_occu preserves partial site occupancies when rebuilding the oriented unit cell, which is what re-enables disordered slab generation. The added test_disordered_slab covers the Na/K case. CI is green. LGTM.