Feature #78: Output system improvements
Replace fEvtHdr.fEvtNum with absolute event counter
If one replays multiple input files in a single analysis session (to add all their events together in the output), duplicate event numbers appear in the event header. This is expected since the event number recorded there is simply the CODA event number of the respective input file, which usually start at 1 for each file. However, what's missing is some sort of continuous counter for the effective event number. Perhaps we should replace fEvtHdr.fEvtNum with this counter value. To keep the original information available, we could add fRawEvtNum (=CODA even number) and fInputFileIdx (input file index). Also, at least the continuous counter should be a 64-bit integer since one might conceivably have more than 2G/4G events.