Add HSV functions to valid tax_func_type

[jdebacker]

This PR updates the default_parameters.json file to add the HSV tax function type (HSV) to the list of valid entries for the tax_func_type parameter. This should have been added in PR #932, but was left out.

In addition, this PR updates the documentation files and some docstrings to reduce the number of warnings Jupyter-Book issues (partially addressing Issue #747).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.92%. Comparing base (c16cf28) to head (f940b5a).
Report is 2 commits behind head on master.

❗ Current head f940b5a differs from pull request most recent head 2e2a85f

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #941      +/-   ##
==========================================
- Coverage   71.93%   71.92%   -0.02%     
==========================================
  Files          19       19              
  Lines        4725     4723       -2     
==========================================
- Hits         3399     3397       -2     
  Misses       1326     1326              

Flag	Coverage Δ
unittests	71.92% <100.00%> (-0.02%)	⬇️

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Files	Coverage Δ
ogcore/__init__.py	100.00% <100.00%> (ø)
ogcore/elliptical_u_est.py	100.00% <ø> (ø)
ogcore/utils.py	86.49% <ø> (ø)

[rickecon]

@jdebacker. This is great. Thanks for fixing all those errors in the documentation. And the HSV addition in the documentation and in the default_parameters.json is nice. Merging.