Fix chemical system method for different oxidation states

[danielzuegner]

Summary

Currently, when the same element occurs in multiple oxidation states in a structure (e.g., Composition({"Na+": 1, "Na2+": 1, "Cl-": 1})), structure.composition.chemical_system yields that element repeated, e.g., Composition({"Na+": 1, "Na2+": 1, "Cl-": 1})), structure.composition.chemical_system == "Cl-Na-Na". My hypothesis is that this was introduced by the fix to this issue: #2248.

This PR fixes the bug by using the newly introduced chemical_system_set property of Composition, and adds the line

assert Composition({"Na+": 1, "Na2+": 1, "Cl-": 1}).chemical_system == "Cl-Na"

to test_composition.py.

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)

[mkhorton]

Thanks for reporting and fixing @danielzuegner, this seems like a clear bug. We should get this merged :)