Restored primary vertex branch in event-based reconstruction. Refs #2096.

With MR310, the primary vertex branch was removed from event-based reconstruction. This created problems, since many analysis tasks do not yet read from the event objects, but access the primary vertex branch (single object) directly. For the RC APR21, a fix was introduced to cope with this. One primary vertex object per tree entry is again created. In case there are several events in the entry, the branch object corresponds to the vertex of the first event.