Solution: fixes failures when oxi state is unknown

[rkingsbury]

Closes #116 by improving handling of unknown oxidation states.

Also bumps pre-commit versions (unrelated)

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 82.28%. Comparing base (02e28bb) to head (934db1f).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/pyEQL/solution.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
+ Coverage   82.24%   82.28%   +0.03%     
==========================================
  Files          10       10              
  Lines        1453     1456       +3     
  Branches      293      293              
==========================================
+ Hits         1195     1198       +3     
  Misses        220      220              
  Partials       38       38              

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