Add satpy cf reader

[mraspaud]

This adds a reader for satpy's own cf files.

• Closes satpy to support reading of satpy generated netcdf cf files #1145
• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented
• Add your name to AUTHORS.md if not there already

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[djhoese]

except (AttributError, KeyError): or is this doing something special?

[mraspaud]

the most indented try shouldn't be needed if we fix the cf writer, so it's not supposed to be there. But yeah, I wanted to distinguish the two errors for now. The KeyError is ok, the AttributeError shouldn't happen at all...

[coveralls]

Coverage decreased (-0.2%) to 89.499% when pulling f3ad83f on mraspaud:feature-satpy-cf-reader into b8bd9ab on pytroll:master.

[codecov]

Codecov Report

Merging #1205 into master will decrease coverage by 0.10%.
The diff coverage is 25.71%.

@@            Coverage Diff             @@
##           master    #1205      +/-   ##
==========================================
- Coverage   89.60%   89.49%   -0.11%     
==========================================
  Files         200      201       +1     
  Lines       29487    29740     +253     
==========================================
+ Hits        26422    26617     +195     
- Misses       3065     3123      +58     

Impacted Files	Coverage Δ
satpy/readers/satpy_cf_nc.py	25.71% <25.71%> (ø)
satpy/readers/avhrr_l1b_gaclac.py	93.33% <0.00%> (-1.34%)	⬇️
satpy/readers/hrit_jma.py	97.94% <0.00%> (-1.22%)	⬇️
satpy/readers/yaml_reader.py	95.30% <0.00%> (-0.13%)	⬇️
satpy/writers/cf_writer.py	93.77% <0.00%> (-0.03%)	⬇️
satpy/writers/geotiff.py	90.74% <0.00%> (ø)
satpy/readers/netcdf_utils.py	100.00% <0.00%> (ø)
satpy/readers/seviri_l1b_native.py	71.96% <0.00%> (ø)
satpy/tests/reader_tests/test_ahi_hrit.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_fci_l1c_fdhsi.py	100.00% <0.00%> (ø)
... and 10 more

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