Fix handling of area creation in the grib reader

[djhoese]

Turns out sometimes grib files have their data flipped. This now catches that.

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

[coveralls]

Coverage decreased (-0.009%) to 73.933% when pulling c404a93 on djhoese:bugfix-grib-areas into 8667ef9 on pytroll:master.

[codecov]

Codecov Report

Merging #484 into master will decrease coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
- Coverage   73.94%   73.93%   -0.01%     
==========================================
  Files         137      137              
  Lines       18217    18222       +5     
==========================================
+ Hits        13470    13472       +2     
- Misses       4747     4750       +3

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_grib.py	95.83% <100%> (+0.08%)	⬆️
satpy/readers/grib.py	80.14% <70%> (-1.75%)	⬇️

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