QA for digi events. Checks the digi time distribution within the event.

This simple QA in the task format checks the (STS) digi time distribution within each constructed event. This distribution should be peaked, demonstrating that real events have been triggered on and were built, and not just some noise is sampled, which would result in a flat distribution.

The corresponding histogram is registered to the THttpServer of FairRunOnline, but I do not yet know how to get it on screen. So, for the time being, it is also written to the output file. Example plots are attached below.

N.b.: I notice that filling the (single) histogram (TH1F) is rather costly: the tasks consumes like 70 ms per timeslice (about 6.5 millions digis / calls to TH1F::Fill()). The task also calculates the first and second moment of the distribution numerically; without histogram filling, the time consumation is only about 10 ms per timeslice.