Use Satpy for SEVIRI calibration

[sfinkens]

Use Satpy for SEVIRI calibration, once pytroll/satpy#2571 has been merged.

[ninahakansson]

The "meirink-2023" can now be used from satpy. However there are new IR calibration done by EUMETSAT. Not sure how these can be combined.

[sfinkens]

I think currently you should be able to do this

eum_coefs = {
    'IR_108': {'gain': 1.234, 'offset': 0.123},
    # etc
}
scene = satpy.Scene(
    filenames,
    reader='seviri_l1b_...',
    reader_kwargs={
        'ext_calib_coefs': eum_coefs,
        'calib_mode': 'meirink-2023'
    }
)

During the last PCW I prepared for channel-specific coefficients, see https://satpy.readthedocs.io/en/stable/api/satpy.readers.utils.html#satpy.readers.utils.CalibrationCoefficientPicker. But I haven't adapted the SEVIRI readers, yet.

[ninahakansson]

From Meirink:
The ‘meirink-2023’ setting in satpy has an offset of 0.50%, 0.39%, and -0.19% for the
VIS006, VIS008 and IR_016 channels, respectively. This is due to an implementation error in
‘meirink-2023’. It uses the correct calibration slopes but the (wrong) nominal calibration
offsets. Instead, calibration offsets should be calculated as -51*cal_slope