More Run-4 preprations for the LArHitEMapToDigitAlg
With the phase-2 upgrade, the LAr-Readout will use only 2 gains instead of 3 so far. The exact gain-switching strategy is not yet decided. The MR prepares the ground by:
- New enum entry CaloGain::LARNGAINR4
- Separate the gain-switching code into a dedicated function
- Some modernization/optimization