Ion: fix CO2- and I3- parsing errors; enhance tests

[rkingsbury]

Summary

My recent PR #3942 contained two bugs that were revealed by testing in downstream code (see pyEQL PR). This PR fixes those and expands the unit tests to catch them.

>>> Ion.from_formula('CO2-').reduced_formula
'C2O4[-0.5]'
>>> Ion.from_formula('CO2--').reduced_formula
'C2O4[-1]'

>>> Ion.from_formula('I3-').get_reduced_formula_and_factor()
('I3', 1.0)
>>> Ion.from_formula('I-').get_reduced_formula_and_factor()
('I3', 0.3333333333333333)

[rkingsbury]

@janosh I request a quick merge and release on this one so that it doesn't adversely affect anyone doing aqueous work (e.g. Pourbaix diagrams) that involve I[-1]. Sorry I didn't catch this in the original PR!

[janosh]

thanks @rkingsbury! 👍