Add HRV full disk navigation for seviri_l1b_native-reader

[ameraner]

PR implements the navigation for the SEVIRI HRV full disk case in the Native format reader.
The HRV channel covers two separated geographical areas; when ordering the full disk data, the two areas are stacked together and need to be navigated to the original correct geolocations by reading the correspondent metadata and defining two area_definitions. This is now implemented for the Native reader, which now covers all data order combinations.

• Closes Add HRV navigation to seviri_l1b_native-reader #975
• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy
• Fully documented
• Add your name to AUTHORS.md if not there already

[mraspaud]

LGTM, thanks for the much needed work with HRV!

[coveralls]

Coverage increased (+0.02%) to 86.887% when pulling 68ec7d2 on ameraner:seviri_hrv_full_disk_navigation_for_native_reader into 177d150 on pytroll:master.

[codecov]

Codecov Report

Merging #985 into master will increase coverage by 0.02%.
The diff coverage is 92.45%.

@@            Coverage Diff             @@
##           master     #985      +/-   ##
==========================================
+ Coverage   86.86%   86.88%   +0.02%     
==========================================
  Files         181      181              
  Lines       27461    27499      +38     
==========================================
+ Hits        23854    23893      +39     
+ Misses       3607     3606       -1

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_seviri_l1b_native.py	93.6% <100%> (+0.27%)	⬆️
satpy/readers/seviri_l1b_native.py	71.64% <87.87%> (+4.4%)	⬆️
satpy/scene.py	90.35% <0%> (-0.18%)	⬇️

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[ameraner]

Added a new commit responding to Sauli's review.

[sjoro]

Looks good to me! Thank you @ameraner for this much needed feature!