Fix nc_nwcsaf_msg reader's handling of projection units

[pnuu]

The reader for NWC SAF GEO was based on NetCDF files with Earth shape (a, b) and satellite altitude h in kilometers, but the area extents in meters. Apparently in some (or most?) cases the projection in proj4_string has these defined properly in meters.

This PR adds a check in which of these proj4_string is in the file.

• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[coveralls]

Coverage increased (+0.2%) to 73.25% when pulling 0d43839 on bugfix-nc_nwcsaf_coordinates into 75e8cb8 on master.

[codecov]

Codecov Report

Merging #446 into master will increase coverage by 0.17%.
The diff coverage is 88.13%.

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
+ Coverage   73.07%   73.25%   +0.17%     
==========================================
  Files         134      135       +1     
  Lines       17662    17716      +54     
==========================================
+ Hits        12907    12977      +70     
+ Misses       4755     4739      -16

Impacted Files	Coverage Δ
satpy/tests/reader_tests/__init__.py	97.36% <100%> (+0.07%)	⬆️
satpy/readers/nc_nwcsaf.py	36.18% <81.25%> (+19.16%)	⬆️
satpy/tests/reader_tests/test_nc_nwcsaf.py	90.47% <90.47%> (ø)

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