Fix offset correction in seviri_l1b_hrit

[sfinkens]

Fix the geo-reference offset correction in seviri_l1b_hrit: Area extent must be shifted in S-E direction (currently N-E). See https://github.com/pytroll/satpy/wiki/SEVIRI-georeferencing-offset-correction for details.

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

[coveralls]

Coverage increased (+0.04%) to 79.009% when pulling 6c47241 on sfinkens:fix-seviri-offset-correction into c95603f on pytroll:master.

[coveralls]

Coverage increased (+0.04%) to 79.009% when pulling a7efc06 on sfinkens:fix-seviri-offset-correction into c95603f on pytroll:master.

[codecov]

Codecov Report

Merging #688 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #688      +/-   ##
=========================================
+ Coverage   78.99%     79%   +0.01%     
=========================================
  Files         138     138              
  Lines       20592   20595       +3     
=========================================
+ Hits        16267   16272       +5     
+ Misses       4325    4323       -2

Impacted Files	Coverage Δ
satpy/readers/seviri_l1b_hrit.py	77.91% <100%> (+1.25%)	⬆️
satpy/tests/reader_tests/test_seviri_l1b_hrit.py	97.9% <100%> (+0.03%)	⬆️
satpy/scene.py	89.71% <0%> (-0.19%)	⬇️

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[mraspaud]

LGTM