Reader for NWCSAF/MSG 2013 format

[adybbroe]

Adding support to read the NWCSAF/MSG v2013 formattet cloud products.

• Closes Unintended/wrong behaviour of getitem method in HDF5FileHandler? #888
• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy
• Fully documented

[coveralls]

Coverage increased (+0.07%) to 86.739% when pulling 462af23 on adybbroe:feature-old-nwcsaf-geo-format into e958d7d on pytroll:master.

[codecov]

Codecov Report

Merging #886 into master will increase coverage by 0.07%.
The diff coverage is 95.02%.

@@            Coverage Diff             @@
##           master     #886      +/-   ##
==========================================
+ Coverage   86.66%   86.73%   +0.07%     
==========================================
  Files         177      179       +2     
  Lines       27036    27252     +216     
==========================================
+ Hits        23431    23638     +207     
- Misses       3605     3614       +9

Impacted Files	Coverage Δ
satpy/composites/__init__.py	78.3% <0%> (-0.1%)	⬇️
satpy/tests/reader_tests/test_hdf5_utils.py	94.02% <100%> (+1.38%)	⬆️
satpy/tests/reader_tests/__init__.py	98.3% <100%> (+0.02%)	⬆️
satpy/readers/hdf5_utils.py	91.93% <90%> (-0.38%)	⬇️
satpy/tests/reader_tests/test_nwcsaf_msg.py	95.31% <95.31%> (ø)
satpy/readers/nwcsaf_msg2013_hdf5.py	96.1% <96.1%> (ø)
satpy/scene.py	90.53% <0%> (+0.17%)	⬆️

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[mraspaud]

Thanks for adding this legacy (but important!) format reader. You're missing tests for the hdf5 utils changes, and for you reader.

[mraspaud]

Does this fix #888 ?

[adybbroe]

@mraspaud I suppose it does fix the old issue, as far as it needs fixing at least...