Cleanup interpolation module and test suite

[tpvasconcelos]

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📚 Documentation preview 📚: https://ridgeplot--269.org.readthedocs.build/en/269/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (54f22df) to head (ad88c65).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #269      +/-   ##
===========================================
+ Coverage   99.24%   100.00%   +0.75%     
===========================================
  Files          13        13              
  Lines         532       537       +5     
  Branches       70        70              
===========================================
+ Hits          528       537       +9     
+ Misses          2         0       -2     
+ Partials        2         0       -2     

Flag	Coverage Δ
combined-src	99.25% <100.00%> (+0.38%)	⬆️

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