Fix aapp_l1b reader to behave nicely on missing datasets

[mraspaud]

This PR fixes the aapp_l1b reader to behave correctly on missing datasets (3a or 3b) and implements the available_datasets method.

• Closes AAPP AVHRR level 1 reader raises a Value error when a channel is missing #1330
• Tests added
• Passes flake8 satpy
• Refactor

[codecov]

Codecov Report

Merging #1333 into master will increase coverage by 0.03%.
The diff coverage is 96.12%.

@@            Coverage Diff             @@
##           master    #1333      +/-   ##
==========================================
+ Coverage   90.24%   90.27%   +0.03%     
==========================================
  Files         222      222              
  Lines       32719    32788      +69     
==========================================
+ Hits        29526    29599      +73     
+ Misses       3193     3189       -4     

Impacted Files	Coverage Δ
satpy/readers/aapp_l1b.py	89.38% <93.75%> (+3.10%)	⬆️
satpy/tests/reader_tests/test_aapp_l1b.py	100.00% <100.00%> (ø)

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[coveralls]

Coverage increased (+0.03%) to 90.274% when pulling 3b7986a on mraspaud:fix-aapp-channels-availability into e77cd29 on pytroll:master.

[mraspaud]

@pnuu could you take a few minutes to test this PR?

[djhoese]

Available datasets looks good to me.