Summary

Note: This code change is no longer required for #4078 and the valences property does not return (idx, ne) for every valence subshell but only every partially filled valence subshell (unless all subshells are filled, which causes only the last subshell to be returned). I don't see a use for this criteria, so I will fix this to behave as intended. I am also okay with letting this PR be closed and abandoned since it is no longer needed for #4078.

Major changes:

• feature 1: For both ElementBase and Species class, the code within the "valence" property was copy-pasted into a new "valences" property, with the error raised for ambiguous valence (more than one valence subshell) removed
  (if len(valence) > 1: raise ValueError(f"{self} has ambiguous valence"))
  and return signature changed from tuple[int | np.nan, int] to list[tuple[int | np.nan, int]] (return valence instead of return valence[0])
• fix 1: To remove redundancy, the "valence" property for ElementBase and Species was re-written to raise the ambiguous valence ValueError if len(self.valences) > 1, and otherwise return self.valences[0]

TODO

• Loosen return criteria for valences to return all valence subshells while maintaining selectivity of valence