Fix VIIRS SDR reading of visible channels at nighttime

[mraspaud]

Since #538, we are depending on the number of scans being read from the files to create the datasets at read time. However, the variable read in that PR is set to a no-data value for visible channels at nighttime.

This PR fixes the problem by accessing another metadata attribute providing a reliable number of scans, even at night time.

• Closes sdr_viirs_l1b reader fails in 0.13, recent master, Works with version 0.12.0 #692
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[TAlonglong]

Thanks.

Yes this works for me on the data that failed previously.

[codecov]

Codecov Report

Merging #693 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #693      +/-   ##
==========================================
+ Coverage   79.14%   79.15%   +<.01%     
==========================================
  Files         140      140              
  Lines       20767    20774       +7     
==========================================
+ Hits        16436    16443       +7     
  Misses       4331     4331

Impacted Files	Coverage Δ
satpy/readers/viirs_sdr.py	85.02% <100%> (+0.27%)	⬆️
satpy/tests/reader_tests/test_viirs_sdr.py	92.3% <100%> (+0.02%)	⬆️

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[coveralls]

Coverage increased (+0.007%) to 79.152% when pulling a891d74 on mraspaud:fix-viirs-visible-night into 3fc5f10 on pytroll:master.

[djhoese]

LGTM