Implements tracking bunch back in time; replace queues for time step size, num time steps and zstop with class that stores all these informations in a list; let the SOURCE element absorbe all particles in case of back tracking; disables automatic centering of FROMFILE distributions;

Use different chains of random numbers on each core of a parallel run in CollimatorPhysics. Furhtermore fix simulation time after all particles were in material and fix spurious structure in longitudinal position with period length of time step that was present after passage of material.

bugfix: increase population by number of worker groups; arguments as members to avoid multiple getarg calls

first version of multi-slit/pepperpot/what ever aka flexible collimator. However way to slow to be practical

This reverts commit 69985bb1.