refactor: make nominal love/shida numbers kwargs

[tsutterley]

refactor: make mass ratios of sun/moon kwargs
refactor: output solid earth corrections as XYZ
docs: add background section detailing methods

[codecov]

Codecov Report

Patch coverage: 82.17% and project coverage change: +6.77 🎉

Comparison is base (fd5e7ff) 61.99% compared to head (ca92293) 68.77%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
+ Coverage   61.99%   68.77%   +6.77%     
==========================================
  Files          39       43       +4     
  Lines        7652     8496     +844     
  Branches      829      964     +135     
==========================================
+ Hits         4744     5843    +1099     
+ Misses       2596     2314     -282     
- Partials      312      339      +27     

Flag	Coverage Δ
unittests	68.77% <82.17%> (+6.77%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyTMD/io/ATLAS.py	41.24% <27.27%> (+0.46%)	⬆️
pyTMD/check_tide_points.py	65.15% <28.57%> (-1.52%)	⬇️
pyTMD/io/GOT.py	64.52% <30.00%> (+0.75%)	⬆️
pyTMD/load_nodal_corrections.py	40.00% <33.33%> (-58.76%)	⬇️
pyTMD/io/OTIS.py	37.34% <35.00%> (+0.24%)	⬆️
pyTMD/io/FES.py	46.05% <35.29%> (+0.38%)	⬆️
pyTMD/compute_tide_corrections.py	22.48% <35.63%> (+2.76%)	⬆️
pyTMD/calc_astrol_longitudes.py	60.00% <42.85%> (-13.18%)	⬇️
pyTMD/time.py	68.22% <52.94%> (+0.56%)	⬆️
pyTMD/io/model.py	58.37% <63.35%> (+34.99%)	⬆️
... and 21 more

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