+hits creation:
+ HMPID uses 2 types fo hits: MIP hit and photon hit. Both types are implemented in the same class AliHMPIDHit.
+ The difference in ctor patterns is only in energy variable: for photon it is full energy, whereas for MIP it's energy lost in amplification gap gas.
+ Hit for photon is created when photon enters the volume representing a single pad of segmeneted photocathode and survives QE test.
+ Hit for MIP is created when MIP exits amplification gap (or disappired for whatever reason). The responsible code is AliHMPIDv1::StepManager().
+ Internally in ctor, the energy is converted to the total charge accamulated for this hit expressed in QCD channels. This is done in a honest manner for photon:
+ photon always produces a single electron, and the response of the chamber to a single electron pulse is parametrized out of test beam data for few HV sets.
+ For MIP the same procedure is generally wrong: the total energy lost by particle is devided by ionization potential, this value is interpreted as number of electrons
+ created, then each electon contribution is sampled according to puasonian distribution with the same single electron pulse mean as fro photons. The final charge is
+ then a sum of all single electon's contributions, it substitutes the value of energy.