AGIPD: support SlopesCS

Looking at pycalibration!894 (merged), trying to add option to choose between PC and CS for relative gain slopes.

• the node corrections.relGainPc is now corrections.relGain
• added corrections.relGain.sourceOfSlopes (string of either "PC" or "CS")
• added corrections.relGain.adjustMgBaseline (whether to apply md_additional_offset; should have maybe always been there)
• I figure that for CS, we can just reuse the corrections.relGain.mdAdditionalOffset value instead of adding another value like csMgAdjust (up for debate, though)
• renaming some of the variables relating to X-ray correction because they were confusing

Have tested superficially that it can query and load some constants and that something happens when switching between PC and CS. Pointing to the big refactor branch because that makes it easier to add this sort of thing.