Add support for NWCSAF GEO v2018, retain support for v2016

[simonrp84]

This PR allows SatPy to read files created by the NWC-SAF's recently released GEO software, version 2018. The main changes are that the ASII-NG module has been split into -TF and -GF components.

The iSHAI module also has quite a few new variables (mainly differences between retrieved and NWP values).

I have also added the status_flag, conditions and quality flags for the majority of data, in case these are useful to the user. This is a repeat of PR #631, but with some changes to allow support for both 2016 and 2018 files. It also doesn't include an erroneous commit on the VIIRS files...

• Tests passed
• Passes flake8 satpy
• Fully documented

[coveralls]

Coverage decreased (-0.02%) to 83.614% when pulling 05a4edc on simonrp84:nwcsaf2018 into 4f63df8 on pytroll:master.

[codecov]

Codecov Report

Merging #820 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #820      +/-   ##
==========================================
- Coverage   83.63%   83.61%   -0.02%     
==========================================
  Files         167      167              
  Lines       24490    24503      +13     
==========================================
+ Hits        20481    20488       +7     
- Misses       4009     4015       +6

Impacted Files	Coverage Δ
satpy/readers/nwcsaf_nc.py	35.18% <100%> (ø)	⬆️
satpy/readers/viirs_edr_active_fires.py	90.74% <0%> (-2.74%)	⬇️
satpy/readers/__init__.py	94.27% <0%> (-0.61%)	⬇️
satpy/tests/test_readers.py	97.97% <0%> (-0.5%)	⬇️
satpy/scene.py	90.01% <0%> (-0.18%)	⬇️
.../tests/reader_tests/test_viirs_edr_active_fires.py	98.72% <0%> (+0.02%)	⬆️
satpy/readers/yaml_reader.py	93.39% <0%> (+0.18%)	⬆️

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[mraspaud]

Thanks for adding all these products, it looks good to me.

[mraspaud]

@simonrp84 As a user, I would be interested in having access to documentation about these product. Do you know of a link to these that we could append to the documentation ?

[simonrp84]

@simonrp84 As a user, I would be interested in having access to documentation about these product. Do you know of a link to these that we could append to the documentation ?

Most of the documentation is here: http://www.nwcsaf.org/web/guest/archive
It's not the easiest interface to navigate, though, and I'm not sure how those docs compare to the ones hidden behind the login screen...

[mraspaud]

Ok, thanks. Can you maybe add it to the nwcsaf module documentation ? I can't think of a better place to put it.

[mraspaud]

Ok, I fixed the link to the documentation so that the PR ready to merge