Add counters for processed number of events, non-empty events and candidates to FunTuple

This issue concerns the monitoring of user jobs (see https://gitlab.cern.ch/lhcb-dpa/project/-/issues/76). In this regard, @erodrigu made following suggestions to FunTuple:

• Event retention rate: Add counters in FunTuple for total number of events processed and total number of non-empty events.
• Storage usage versus retention rate: A large ratio might indicate the abuse of storage with lots of irrelevant information. Suggested to histogram the per-event candidate multiplicity in FunTuple.

The first point can be implemented. Regarding the second point, I suggested we just add a counter for number of events with per-event candidate muliplicity > 1? FunTuple writes nTuple on candidate-by-candidate basis, so histogram of event-by-event info is not there. I guess such a functionality will only used for monitoring. It can also be calculated "offline" (i.e. from the nTuple) once we store the eventNumber and runNumber info for the candidate.