[AGIPD][DARK] Correctly retrieve old constants for comparison

Part of https://git.xfel.eu/gitlab/calibration/planning/issues/56

This merge request is way longer than it should be because I messed up when merging https://git.xfel.eu/gitlab/detectors/pycalibration/merge_requests/478 - I developed that one against https://git.xfel.eu/gitlab/detectors/pycalibration/merge_requests/465 and I forgot to switch when merging. So https://git.xfel.eu/gitlab/detectors/pycalibration/merge_requests/465 was merged into master and then https://git.xfel.eu/gitlab/detectors/pycalibration/merge_requests/478/diffs was merged into https://git.xfel.eu/gitlab/detectors/pycalibration/merge_requests/465, never reaching master.

The plots were "fortunately" broken anyway by the erroneous old constant retrieval which is what this MR is actually supposed to fix. I've added comments to point out the two actual new things in this MR. The changes from https://git.xfel.eu/gitlab/detectors/pycalibration/merge_requests/478/diffs were reasonably tested back when I originally intended to merge it.