Draft: LPD parallel gain

#52

Challenge:

• Input data has 3 x the amount of frames (all frames repeated for each gain stage)
• Cell table and pulse table have size corresponding to the input data, but second two thirds are garbage (just duplicate first third twice)
• Output has one third the frames of input because we threshold and pick out

This doesn't fit super well into the way the kernel runners are currently structured (see the diff). One issue:

• It should be a correction step because those have parameters which can be tweaked
• It should be a constant parameter because that is something which applies regardless of corrections
  • Even if we never use it for querying constants, it would, like gain mode for AGIPD, be useful for the device to know this about operation mode
  • Correction steps can be turned on / off for full and preview separately, but parallel gain should be on or off

After I figure that out, another request to consider: provide preview of each of the gain stages independently.

• Add extra corrected preview output channels, only to be used for parallel gain mode?
• Figure out how this fits in; worst case redefine correct entirely for LpdBaseRunner...