code quality improvements in preparation for v0.3.1 release

[jpvantassel]

In this pr:

• 📚 Improvements to documentation.
• 🔨 Various refactoring tweaks.
• ✅ Increased test coverage.

[codecov-commenter]

Codecov Report

Merging #1 into master will increase coverage by 0.22%.
The diff coverage is 96.38%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   92.78%   93.01%   +0.22%     
==========================================
  Files          13       13              
  Lines        1539     1560      +21     
==========================================
+ Hits         1428     1451      +23     
+ Misses        111      109       -2     

Impacted Files	Coverage Δ
swprepost/parameterization.py	80.97% <50.00%> (ø)
swprepost/groundmodel.py	97.47% <91.30%> (-0.69%)	⬇️
swprepost/curve.py	100.00% <100.00%> (ø)
swprepost/curveuncertain.py	97.91% <100.00%> (+0.48%)	⬆️
swprepost/dispersionset.py	98.96% <100.00%> (+0.02%)	⬆️
swprepost/dispersionsuite.py	98.43% <100.00%> (-0.10%)	⬇️
swprepost/groundmodelsuite.py	98.23% <100.00%> (-0.09%)	⬇️
swprepost/suite.py	100.00% <100.00%> (+6.45%)	⬆️
swprepost/target.py	99.32% <100.00%> (-0.02%)	⬇️

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