Flatfielding factor instead of absolute & relative factor for amplitude correction

[StFroese]

closes #1397

[Hckjs]

Was it on purpose to set this PR to draft again? @maxnoe

[kosack]

I added some comments to the linked issue, but I disagree that these should be separated. Flat-fielding and gain correction are not the same thing: even if you can combine them, you are losing important information that is needed for data quality monitoring as well as for better reconstruction techniques that do not use the flatfielding information but do use the absolute gains.

[maxnoe]

: even if you can combine them, you are losing important information that is needed for data quality monitoring as well as for better reconstruction techniques that do not use the flatfielding information but do use the absolute gains.

The change here and discussed in #1397 is not about the monitored quantities, it's about what is filled into the event so that it can be applied in the CameraCalibrator.

I argued that the event should only contain a single correction coefficient, that is computed from all the different monitoring inputs we have for that specific event.