LPD: support parallel gain mode

Discussed during group meeting 12.07.2024, some thoughts:

• [recap technical details of what parallel gain mode is]
  • https://euxfel-da.zulipchat.com/#narrow/stream/258300-calibration/topic/LPD.20parallel.20gain/near/450165027
  • E-mail thread
• It seems the gain bits are still in data stream and correct -> can probably use existing correction kernel
• Plan:
  • Fix up cell table (see how EXtra-data does it)
  • Run regular correction kernel, correcting all three versions in their respective gain stages
  • Provide previews for each
  • Run extra thresholding kernel
    • AGIPD-style: start low, if exceed threshold, go medium, if exceed other threshold, go high
    • Slice down the cell and pulse tables as we now have the actual number of frames
• Open question: will we ever need to send corrected but un-thresholded (i.e. all three gains) downstream?
  • Should be toggleable like any other correction step, I think
  • But then "fix up cell table" and "threshold and select gain per pixel" are two different steps (if subconfig of one step, then this is iffy)

I'm also reminded of #37 (closed)

Have set due date to one week before user experiment 20-25 Nov, but should get something up and running ASAP.