Add GOES-17 support to the 'geocat' reader

[djhoese]

The 'geocat' group at the SSEC is now working on producing files operationally with GOES-17 data. I had hacked something together for GOES-16, but forgot to include GOES-17. This extends the hack to support GOES-17 until geocat adds projection information to their files (possibly in the future?).

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

[coveralls]

Coverage increased (+0.01%) to 88.112% when pulling b4e33b9 on djhoese:bugfix-geocat-g17 into fbefad6 on pytroll:master.

[codecov]

Codecov Report

Merging #1067 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1067      +/-   ##
==========================================
+ Coverage   88.09%   88.11%   +0.01%     
==========================================
  Files         191      191              
  Lines       29439    29450      +11     
==========================================
+ Hits        25935    25949      +14     
+ Misses       3504     3501       -3

Impacted Files	Coverage Δ
satpy/readers/geocat.py	88.46% <ø> (+0.64%)	⬆️
satpy/tests/reader_tests/test_geocat.py	100% <100%> (+3.03%)	⬆️
satpy/scene.py	89.73% <0%> (-0.18%)	⬇️

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[djhoese]

Got slack approval from @mraspaud. Merging...