Allow channel-specific calibration mode for SEVIRI

[sfinkens]

Feature Request

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

Since #2589 there are three calibration modes available for SEVIRI (Nominal, GSICS, Meirink). But currently it is not possible to mix Meirink (VIS) and GSICS (IR), because there's only one global calibration mode for all channels.

satpy.Scene(..., reader_kwargs={"calib_mode": "meirink-2023"})

If the requested coefficients are not available for a certain channel, the reader falls back to nominal coefficients.

Describe the solution you'd like

Allow users to specify the calibration mode per channel:

satpy.Scene(..., reader_kwargs={"calib_mode": {"VIS006": "meirink-2023", "IR_108": "GSICS"}})

For other channels fall back to Nominal.

Describe any changes to existing user workflow

None, if both forms are allowed:

{"calib_mode": "GSICS"}  # global
{"calib_mode": {"VIS006": "meirink-2023", "IR_108": "GSICS"}  # per channel

Additional context
None

[gerritholl]

Per channel, or per calibration mode? If loading all solar channels with one calibration and all terrestrial with another, it's a bit cumbersome to have to write out all channels. Maybe {"calib_mode": {"reflectance": "meirink-2023", "brightness_temperature": "GSICS"}}, if that makes sense? Then in the (unlikely?) case that people want to mix calibration modes for channels calibrated to the same quantity (such as different solar channels), only then they have to specify the channels explicitly.

Also, this should be implemented in a way that it can potentially be used for any reader and not only for SEVIRI.

[sfinkens]

Good suggestion, thanks @gerritholl!

[mraspaud]

I think that's a good idea, and I like the addendum by @gerritholl also. So please go ahead :)