Generalize gain-handling in LArHitEMapToDigitAlg
Generalizes the handling of the readout gains (HIGH,MEDIUM,LOW) in the LArHitEMapToDigitAlg. Add new properties to define which gains are to be used for EM, EMIW, HEC, FCAL. This should accomodate also the Run-4 situation with only 2 readout gains.