Fix nucaps reader for NUCAPS EDR v2 files

[djhoese]

Includes fix for loading ancillary variables when the ancillary variable is being loaded alongside the dataset that needs it. Also fixes handling of CF standard ancillary variable attributes with multiple variables listed. They were comma-separated, but space separated seems to be the standard: http://cfconventions.org/Data/cf-conventions/cf-conventions-1.6/build/cf-conventions.html#ancillary-data

Additionally this fixes #244 which was mainly worried about the instrument name attribute which changed from 'CrIS' to 'CrIS, ATMS, VIIRS' from v1 to v2 of the NUCAPS EDR files.

• Closes Fix NUCAPS reader for NUCAPS EDR v2 files #244
• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[coveralls]

Coverage increased (+0.04%) to 68.744% when pulling 7aaa78b on djhoese:bugfix-nucaps-edr-v2 into 377b5ba on pytroll:master.

[codecov]

Codecov Report

Merging #326 into master will increase coverage by 0.03%.
The diff coverage is 89.28%.

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
+ Coverage   68.73%   68.77%   +0.03%     
==========================================
  Files         121      121              
  Lines       15134    15146      +12     
==========================================
+ Hits        10402    10416      +14     
+ Misses       4732     4730       -2

Impacted Files	Coverage Δ
satpy/readers/file_handlers.py	89.79% <ø> (+1.56%)	⬆️
satpy/readers/netcdf_utils.py	95.38% <100%> (+0.22%)	⬆️
satpy/tests/reader_tests/test_nucaps.py	97.9% <100%> (+0.01%)	⬆️
satpy/readers/yaml_reader.py	85.25% <80%> (+0.21%)	⬆️
satpy/readers/nucaps.py	94.14% <88.88%> (+0.32%)	⬆️

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