Fix geostationary axis handling in CF writer

[djhoese]

This is a solution suitable for a 0.19.1 release since it still works on python 2.7 and doesn't require pyproj 2.0+. See #1038 for the 0.20 long term solution.

This PR does two main things:

1. Get the a/b parameters for the geostationary projection by using pyresample's utilities. This is important for pyproj 2.0+ where a/b combinations may be replaced by their associated datum/ellipsoid name (meaning a/b are not available from the PROJ dict).
2. Fixes the tests so they actually check the attributes.

• Closes NetCDF (CF) writer doesn't include semi_minor_axis/semi_major_axis for new versions of pyproj #1029
• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.02%) to 87.312% when pulling 9ba406e on djhoese:bugfix-cf-projections2 into c2de7cc on pytroll:master.

[codecov]

Codecov Report

Merging #1040 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1040      +/-   ##
==========================================
+ Coverage   87.29%   87.32%   +0.03%     
==========================================
  Files         188      188              
  Lines       28715    28736      +21     
==========================================
+ Hits        25066    25094      +28     
+ Misses       3649     3642       -7

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_cf.py	98.55% <100%> (+0.05%)	⬆️
satpy/writers/cf_writer.py	92.65% <100%> (+1.97%)	⬆️
satpy/scene.py	89.8% <0%> (+0.17%)	⬆️

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[mraspaud]

LGTM