Add utility functions for generating GeoViews plots

[djhoese]

This is a replacement of #541 after git wreaked havoc on @BENR0's code.

• Closes Add utility functions for generating GeoViews plots #541
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[djhoese]

I made this by taking the master branch and cherry picking individual commits related to the geoviews stuff.

[coveralls]

Coverage increased (+0.1%) to 77.844% when pulling 111a9ee on djhoese:geoviews_cherry into ee1ee7d on pytroll:master.

[codecov]

Codecov Report

Merging #567 into master will increase coverage by 0.01%.
The diff coverage is 82.92%.

@@            Coverage Diff             @@
##           master     #567      +/-   ##
==========================================
+ Coverage   77.73%   77.75%   +0.01%     
==========================================
  Files         136      136              
  Lines       19290    19370      +80     
==========================================
+ Hits        14996    15062      +66     
- Misses       4294     4308      +14

Impacted Files	Coverage Δ
satpy/tests/test_scene.py	99.34% <100%> (+0.01%)	⬆️
satpy/multiscene.py	75.51% <27.27%> (-2.87%)	⬇️
satpy/scene.py	84.23% <86.04%> (+0.09%)	⬆️
satpy/tests/reader_tests/test_ahi_hsd.py	96.51% <0%> (ø)	⬆️

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[djhoese]

I'm really confused at how coverage went up when there are no tests added. I'll have to add tests later.