BUG: Fix pickling for CRS builder classes

[snowman2]

• Closes Pickle fails to load Pyproj objects #897
• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[codecov]

Codecov Report

Merging #901 (316b933) into master (cefc33d) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #901      +/-   ##
==========================================
+ Coverage   95.22%   95.29%   +0.06%     
==========================================
  Files          21       21              
  Lines        1677     1680       +3     
==========================================
+ Hits         1597     1601       +4     
+ Misses         80       79       -1     

Impacted Files	Coverage Δ
pyproj/crs/crs.py	97.71% <100.00%> (+0.24%)	⬆️

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