Add grouping parameters for the 'viirs_sdr' reader

[djhoese]

This allows someone to use the group_files utility function or MultiScene.from_files class method with a series of VIIRS SDR files and have them be sorted by orbit and platform. This differs from what would normally be done where only start_time and maybe platform would be used. I made the assumption that most people would want to group orbits together instead of individual granules and I wanted that to work by default. So as an example:

groups = group_files(self.noaa20_files + self.npp_files, reader='viirs_sdr')

Would give you two groups, one for the NOAA-20 files and one for the NPP files. You could also give it two orbits of either one of these platforms. You could get around this by doing something like:

        groups = group_files(self.noaa20_files + self.npp_files, reader='viirs_sdr',
                             time_threshold=60 * 60 * 2,
                             group_keys=('start_time', 'orbit', 'platform_shortname'))

I'm open to discussion of making the start_time version the default and requiring people to use time_threshold to get "chunks" of data.

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

[mraspaud]

LGTM

[codecov]

Codecov Report

Merging #816 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #816      +/-   ##
==========================================
+ Coverage   83.72%   83.74%   +0.01%     
==========================================
  Files         165      165              
  Lines       24115    24139      +24     
==========================================
+ Hits        20190    20214      +24     
  Misses       3925     3925

Impacted Files	Coverage Δ
satpy/tests/test_readers.py	98.46% <100%> (+0.09%)	⬆️

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[coveralls]

Coverage increased (+0.03%) to 83.74% when pulling 14edc87 on djhoese:feature-viirs-sdr-groups into 5b74266 on pytroll:master.