Fix handling of HRV channel navigation for RSS data in seviri_l1b_native reader

[ameraner]

This PR fixes the navigation of the HRV channel from Rapid Scanning Service data in the SEVIRI native format reader.

The issue was caused by the wrong assumption that the HRV RSS data is formatted the same way as HRV ROI data. This was causing wrong navigations, with stretched and shifted images.

Instead, the RSS HRV is handled similarly to the full-disk case; the navigation still needs to be done using the "Actual" lines and columns positions stored in the trailer information. The difference w.r.t. the full-disk case is that only one area is present, instead of two separate ones.

• Tests added
• Tests passed
• Passes flake8 satpy

[ameraner]

vis_sharpened_ir images
Left: current master, Right: with this PR

Notice, in the left image, the mismatch between the coloring extracted from the IR channels and the HRV background.

[coveralls]

Coverage decreased (-0.03%) to 90.008% when pulling d9a008f on ameraner:add_RSS_HRV_navigation into a8fcba6 on pytroll:master.

[codecov]

Codecov Report

Merging #1262 into master will decrease coverage by 0.02%.
The diff coverage is 83.63%.

@@            Coverage Diff             @@
##           master    #1262      +/-   ##
==========================================
- Coverage   90.03%   90.01%   -0.03%     
==========================================
  Files         218      218              
  Lines       31275    31315      +40     
==========================================
+ Hits        28160    28187      +27     
- Misses       3115     3128      +13     

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_seviri_l1b_native.py	92.21% <79.06%> (-2.96%)	⬇️
satpy/readers/seviri_l1b_native.py	72.28% <100.00%> (+0.31%)	⬆️
satpy/readers/__init__.py	93.90% <0.00%> (-1.22%)	⬇️

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[mraspaud]

LGTM, and well done with the simple fix!