REF: Make CRS methods inheritable

[snowman2]

• Closes CRS classmethods changed to staticmethods #847
• Tests added (TODO: More tests)
• Fully documented, including history.rst for all changes and api/*.rst for new API

Notes:

• The from_* methods were changed to bypass the __init__ method. It also validates that it creates the expected type (Idea from CRS classmethods changed to staticmethods #847 (comment)).
• The CRS properties were overridden to return a CRS. The reason for this is because the type of the CRS returned from that type is not guaranteed to always be the type of the original CRS.

[codecov]

Codecov Report

Merging #902 (ea2cc18) into master (43b7cff) will increase coverage by 0.06%.
The diff coverage is 98.80%.

@@            Coverage Diff             @@
##           master     #902      +/-   ##
==========================================
+ Coverage   95.29%   95.35%   +0.06%     
==========================================
  Files          21       21              
  Lines        1680     1724      +44     
==========================================
+ Hits         1601     1644      +43     
- Misses         79       80       +1     

Impacted Files	Coverage Δ
pyproj/crs/crs.py	97.71% <98.80%> (+<0.01%)	⬆️

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