Harmonize SEVIRI area definitions

[sfinkens]

Feature Request

Is your feature request related to a problem? Please describe.

As described by @ameraner in #1186 , the SEVIRI readers provide slightly different area extents:

Reader	north up	lower left x	lower left y	upper right x	upper right y	width/height
nc	yes	-5570248.477339745	-5567248.074173927	5567248.074173927	5570248.477339745	11137496.551513672
hrit	no	5567248.28340708	5570248.686685662	-5570248.686685662	-5567248.28340708	11137496.970092742
hrit (pre 2018)	no	5568748.28340708	5568748.686685662	-5568748.686685662	-5568748.28340708	11137496.970092742
native	no	5567248.074173927	5570248.477339745	-5570248.477339745	-5567248.074173927	11137496.551513672
native (pre 2018)	no	5568748.275756836	5568748.275756836	-5568748.275756836	-5568748.275756836	11137496.551513672

So there are two different image extents (absolute width/height) among the readers. Also, the georeferencing offset correction is exactly 1500 meters in the HRIT reader (hard coded somewhere), whereas it is 1500.2 meters (half a pixel) in the native reader. The MSG Level 1.5 Image Data Format Description (link) says:

The problem is a constant shift of the image by 0.5 low resolution pixels North plus 0.5 low
resolution pixels West. The HRV channel appears shifted by 1.5 HRV pixels North plus 1.5
HRV pixels West.

So the implementation in the native reader, shifting the image by pixel amounts and not hardcoded exact meters, is the most correct.

Describe the solution you'd like
Check whether the image data in the different data formats is identical. If so, find out which area extent is correct and make all SEVIRI readers return that extent (in terms of values; order may differ as we want to keep the orientation of the original data). For the flipped data, a north_up reader kwarg would be nice, to make the image upright.

Describe any changes to existing user workflow
None.

Additional context
There is ongoing work on the utility function for computing the geostationary area extents: #1179

[ameraner]

As for the comment about the possibility of flipping data upright, this was implemented in #1194 as a load kwarg, e.g.: scn.load(['ir_105'], upper_right_corner='NE')

[ameraner]

Fixing this would also benefit the quest for standardising geos AreaDefinition (names) in #1248 :)

[sfinkens]

Fixed (including documentation) by #1485