ENH: Added pyproj.crs.GeocentricCRS

[snowman2]

• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[codecov]

Codecov Report

Merging #903 (5e23b2c) into master (4cde3f4) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #903      +/-   ##
==========================================
+ Coverage   95.35%   95.37%   +0.01%     
==========================================
  Files          21       21              
  Lines        1724     1729       +5     
==========================================
+ Hits         1644     1649       +5     
  Misses         80       80              

Impacted Files	Coverage Δ
pyproj/crs/__init__.py	100.00% <ø> (ø)
pyproj/crs/crs.py	97.74% <100.00%> (+0.02%)	⬆️

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