Add ABI Meso demo data case and clean up documentation

[djhoese]

This adds another GOES-16 ABI case of multiple mesoscale sector over hurricane florence. I also cleaned up a few things that looked wrong or had warnings when generating sphinx documentation.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[djhoese]

I'm not super happy with the way I had to do some of this stuff to get it to work. I don't know if there is a much better or easier way given the limitation on GCP's glob patterns and the way gcsfs works.

[coveralls]

Coverage increased (+0.07%) to 79.829% when pulling 41867c4 on djhoese:feature-abi-meso-demo into 6cabadd on pytroll:master.

[codecov]

Codecov Report

Merging #707 into master will increase coverage by 0.07%.
The diff coverage is 98.66%.

@@            Coverage Diff             @@
##           master     #707      +/-   ##
==========================================
+ Coverage   79.75%   79.82%   +0.07%     
==========================================
  Files         147      147              
  Lines       21300    21407     +107     
==========================================
+ Hits        16988    17089     +101     
- Misses       4312     4318       +6

Impacted Files	Coverage Δ
satpy/tests/test_writers.py	97.81% <ø> (ø)	⬆️
satpy/readers/viirs_edr_active_fires.py	90% <ø> (ø)	⬆️
satpy/readers/viirs_sdr.py	85.02% <ø> (ø)	⬆️
satpy/demo/_google_cloud_platform.py	95.45% <100%> (ø)
satpy/tests/test_demo.py	96.07% <100%> (+2.23%)	⬆️
satpy/demo/__init__.py	95.12% <95.65%> (-0.34%)	⬇️
satpy/resample.py	87.08% <0%> (-0.38%)	⬇️
satpy/tests/test_scene.py	99.42% <0%> (-0.08%)	⬇️
satpy/scene.py	89.89% <0%> (-0.02%)	⬇️

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[mraspaud]

It's not that bad!