[Digitization] Make run start time configurable

Frederic Julian Linzrequested to merge
f.kornas/cbmroot:dT into master

This MR make the run start time configurable, i.e. in time-based mode, the time of the first timeslice can be set by the user (prior always default =0), related to request in https://redmine.cbm.gsi.de/issues/4027

Refs #4027

Edited by Volker Friese

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