mCBM event builders improvements:

* in standard event builder, fill the histo for time difference between digis
* in Time Window event builder algo, improves the SetReferenceDetector method (safer and more verbose)
* in both event builders, add possibility to set a maximum nb of digi cut per detector + small optimizations in trigger check
* in time window event builder, remove the setters for individual min digi nb and keep only the generic one
* in standard event builder, add doxygen description to max digi nb members and to Trigger condition setters
* in time window event builder, add a new internal method to make trigger check more generic