changing constructor-methods from staticmethods to classmethods

[nmaxwell]

changes use of staticmethods to classmethods in constructor-methods in Transformer and CRS. I left from_cf in CRS, as there is some logic in there on whether to return a CRS, a GeographicCRS, a DerivedGeographicCRS, or a CompoundCRS. It may require some rethinking on how handle that properly with classmethods.

• [ x] Closes CRS classmethods changed to staticmethods #847
• [ x] Tests added

[codecov]

Codecov Report

Merging #848 (7fa6dd7) into master (1117367) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 7fa6dd7 differs from pull request most recent head 7be584c. Consider uploading reports for the commit 7be584c to get more accurate results

@@           Coverage Diff           @@
##           master     #848   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files          21       21           
  Lines        1672     1672           
=======================================
  Hits         1592     1592           
  Misses         80       80           

Impacted Files	Coverage Δ
pyproj/crs/crs.py	97.47% <100.00%> (ø)
pyproj/transformer.py	95.93% <100.00%> (ø)

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[snowman2]

Thanks for taking a stab at this 👍 . The issue has been addressed by #902.