docs: add references to new PERTH5 software from Richard Ray

[tsutterley]

feat: add wrapper function for normalizing phase angles
feat: add functions to convert to and from Degree-Minutes-Seconds (DMS)
fix: assert that data type is a known value

[codecov]

Codecov Report

Attention: Patch coverage is 91.22807% with 5 lines in your changes missing coverage. Please review.

Project coverage is 69.44%. Comparing base (76a288a) to head (4800829).

Files	Patch %	Lines
pyTMD/compute.py	50.00%	3 Missing ⚠️
pyTMD/predict.py	0.00%	1 Missing ⚠️
pyTMD/spatial.py	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #305      +/-   ##
==========================================
+ Coverage   69.34%   69.44%   +0.09%     
==========================================
  Files          39       39              
  Lines        8844     8881      +37     
  Branches     1195     1195              
==========================================
+ Hits         6133     6167      +34     
- Misses       2325     2328       +3     
  Partials      386      386              

Flag	Coverage Δ
unittests	69.44% <91.22%> (+0.09%)	⬆️

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