Make filewriter write data on an event-by-event basis

With this MR, the Filewriter writes the data into the TTrees on an event-by-event basis. For each processed event, detector and object type, one vector of objects is stored instead of a continuous stream of the objects. The latter lead to an incapability of merging all hits in a certain event.

As a consequence, in some post-processing macro calling TTree->GetEntry(eventNumber) yields a vector of the requested object type.