Some time in the past, the simple fixed momentum setting for particle gun was lost.

Now we set a fixed energy if the Energy property is set, otherwise we
use momentumMin and momentumMax to chose a momentum from a range.
Also changed some printouts to show the range we chose and the final
value used during simulation.

Implement fast simulation shower modelling. Simplify GFlash to use the same sensitive actions as fast simulation

Co-Authored-By: Andre Sailer <andre.philippe.sailer@cern.ch>