feat: add spherical harmonic functions

[tsutterley]

refactor: move generic astro functions to math.py
feat: add function to calculate altitude, azimuth and D
test: add test for unnormalized legendre polynomials
test: moved normalize_angle test to test_math.py
test: add ECEF to altaz test at USNO

[codecov]

Codecov Report

Attention: Patch coverage is 85.95041% with 17 lines in your changes missing coverage. Please review.

Project coverage is 70.96%. Comparing base (3d34f83) to head (01a6094).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
pyTMD/math.py	76.92%	13 Missing and 2 partials ⚠️
pyTMD/spatial.py	83.33%	1 Missing ⚠️
test/test_math.py	95.23%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #362      +/-   ##
==========================================
+ Coverage   70.90%   70.96%   +0.06%     
==========================================
  Files          41       43       +2     
  Lines       10391    10482      +91     
  Branches     1130     1137       +7     
==========================================
+ Hits         7368     7439      +71     
- Misses       2562     2580      +18     
- Partials      461      463       +2     

Flag	Coverage Δ
unittests	70.96% <85.95%> (+0.06%)	⬆️

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