Ion: handle dissolved gas formulas

[rkingsbury]

Summary

Ion.reduced_formula does not correctly handle dissolved gas molecules, such as O2, for example:

>>> Ion.from_formula('O2').get_reduced_formula_and_factor()
('O', 2)
>>> Ion.from_formula('O2').formula
'O2 (aq)'
>>> Ion.from_formula('O2').reduced_formula
'O(aq)'

This PR modifies Ion.get_reduced_formula_and_factor to correctly report dissolved formulas of dissolved gases H2, N2, O2, F2, Cl2, and O3. It also updates the docstrings of formula and reduced_formula.

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.

[rkingsbury]

@materialsproject/pymatgen-maintainers there are a handful of test failures here that seem unrelated to the changes in this PR. I believe I am up to date with the latest master branch; please advise if further changes are needed. Thanks!

[janosh]

Looks great, thanks @rkingsbury! 👍