Fix unit-aware arithmetics

[coroa]

Fixes #35

• Add tests
• Add documentation

[codecov-commenter]

Codecov Report

Patch coverage: 55.55% and project coverage change: -0.66% ⚠️

Comparison is base (8ee04da) 77.53% compared to head (3744529) 76.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
- Coverage   77.53%   76.87%   -0.66%     
==========================================
  Files           9        9              
  Lines         681      692      +11     
  Branches      176      183       +7     
==========================================
+ Hits          528      532       +4     
- Misses        125      132       +7     
  Partials       28       28              

Files Changed	Coverage Δ
src/pandas_indexing/arithmetics.py	42.30% <42.85%> (-4.21%)	⬇️
src/pandas_indexing/accessors.py	90.00% <100.00%> (+0.20%)	⬆️

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