Fix handling of navigation in a grib file with lons greater than 180

[djhoese]

Grib files are weird. This should hopefully handle more of them a little better.

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

[coveralls]

Coverage increased (+0.07%) to 74.102% when pulling dd60146 on djhoese:bugfix-grib-area into be0c391 on pytroll:master.

[codecov]

Codecov Report

Merging #476 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #476      +/-   ##
=========================================
+ Coverage   74.02%   74.1%   +0.07%     
=========================================
  Files         136     136              
  Lines       18004   18032      +28     
=========================================
+ Hits        13328   13362      +34     
+ Misses       4676    4670       -6

Impacted Files	Coverage Δ
satpy/readers/grib.py	81.88% <100%> (+6.29%)	⬆️
satpy/tests/reader_tests/test_grib.py	95.74% <100%> (+0.93%)	⬆️

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