Add VII L1b netCDF-reader.

[sjoro]

Add EPS-SG VII L1b netCDF reader. Replaces PR #1069 and is based on a most recent master-branch.

• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[mraspaud]

Looks good, provided vii_base_nc is fixed according to the comments in #1219

[coveralls]

Coverage increased (+0.1%) to 90.031% when pulling a0aff96 on sjoro:vii_l1b into 5bf9235 on pytroll:master.

[mraspaud]

try to trigger travis

[codecov]

Codecov Report

Merging #1218 into master will increase coverage by 0.09%.
The diff coverage is 97.53%.

@@            Coverage Diff             @@
##           master    #1218      +/-   ##
==========================================
+ Coverage   89.93%   90.03%   +0.09%     
==========================================
  Files         210      216       +6     
  Lines       30792    31197     +405     
==========================================
+ Hits        27692    28087     +395     
- Misses       3100     3110      +10     

Impacted Files	Coverage Δ
satpy/readers/vii_l1b_nc.py	96.07% <96.07%> (ø)
satpy/tests/reader_tests/test_vii_l1b_nc.py	97.18% <97.18%> (ø)
satpy/tests/reader_tests/test_vii_base_nc.py	97.46% <97.46%> (ø)
satpy/readers/vii_base_nc.py	97.89% <97.89%> (ø)
satpy/readers/netcdf_utils.py	100.00% <100.00%> (ø)
satpy/readers/vii_utils.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_netcdf_utils.py	95.60% <100.00%> (+0.14%)	⬆️
satpy/tests/reader_tests/test_vii_utils.py	100.00% <100.00%> (ø)
... and 4 more

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