Fix CRS object being recreated when adding CRS coordinate

[djhoese]

While fixing other CRS issues with trollimage and pyresample, I noticed that the CRS coordinate being added by Satpy is always created from the proj_str attribute of the AreaDefinition. This is an inaccurate/lossy way of creating a CRS definition. This PR makes it so we just copy the CRS object from the AreaDefinition if it exists. When this functionality was originally added the AreaDefinition didn't have a 'crs' property so creating from the proj_str made a little more sense. Now that we have a 'crs' object in newer versions of pyresample we should use it.

NOTE: I had uninstalled the pre-commit checks so I have not updated resample.py if it needs it.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.005%) to 85.289% when pulling 1b022f2 on djhoese:bugfix-resample-crs-coord into 35739e4 on pytroll:master.

[codecov]

Codecov Report

Merging #915 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #915      +/-   ##
==========================================
+ Coverage   85.28%   85.44%   +0.16%     
==========================================
  Files         174      174              
  Lines       25822    25875      +53     
==========================================
+ Hits        22022    22110      +88     
+ Misses       3800     3765      -35

Impacted Files	Coverage Δ
satpy/tests/test_resample.py	98.37% <100%> (+0.04%)	⬆️
satpy/resample.py	92.97% <100%> (+0.02%)	⬆️
satpy/scene.py	90.65% <0%> (+0.17%)	⬆️
satpy/readers/avhrr_l1b_gaclac.py	91.54% <0%> (+1.4%)	⬆️
satpy/tests/reader_tests/test_nwcsaf_nc.py	95.89% <0%> (+5.41%)	⬆️
satpy/readers/nwcsaf_nc.py	55.15% <0%> (+20.18%)	⬆️

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[mraspaud]

LGTM