Upgrade of dyn. ineff. feature to handle cases when PU is 0

A small improvement to pixel digitizer's dynamic inefficiency feature.
It automatically disables dynamic inefficiency simulation in case when there is zero pile-up per event.

Details:
The _eventPileUp variable gets the pile-up number per event from PileupMixinContent's TrueInteractionList (calculateInstlumiFactor () method), then it is used in pixel_inefficiency () method to check whether the actual pile-up is zero. If it is, then the columnEfficiency remains unchanged, i.e. no dynamic inefficiency is used. 
use TrueInteractionList.at(p); only for code debugging
here use
TrueInteractionList[p];