Fix MERSI-2 and VIRR readers being recognized by pyspectral

[djhoese]

This PR covers just about every category. It is a fix and an enhancement. This includes the changes from #884. Basically, I needed MERSI-2 and VIRR readers to work with rayleigh correct.

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

[coveralls]

Coverage decreased (-0.001%) to 84.644% when pulling bb07fb3 on djhoese:bugfix-virr-mersi2 into f9d0022 on pytroll:master.

[codecov]

Codecov Report

Merging #890 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #890      +/-   ##
==========================================
+ Coverage   84.64%   84.64%   +<.01%     
==========================================
  Files         171      171              
  Lines       25035    25033       -2     
==========================================
- Hits        21191    21190       -1     
+ Misses       3844     3843       -1

Impacted Files	Coverage Δ
satpy/readers/mersi2_l1b.py	96.38% <ø> (-0.09%)	⬇️
satpy/tests/reader_tests/test_virr_l1b.py	97.19% <100%> (ø)	⬆️
satpy/readers/virr_l1b.py	98.41% <100%> (ø)	⬆️
satpy/readers/avhrr_l1b_gaclac.py	91.54% <0%> (+1.4%)	⬆️

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