JEventSource improvements

[nathanwbrei]

This is a place to aggregate problems with JEventSources.

• nskip doesn't do the correct thing when barrier events are present. The user must decide between accurately replaying barrier events and calling a user-defined random-access Skip() callback.
• JEventSource::Preprocess should be renamed
• Multi-level event sources would be highly appreciated
• Event sources should print a report about events skipped/emitted/processed similar to what JEventSource_EVIOpp does (see issue Number of events doesn't match. #429) and discussed in depth in Feature request - cmd line arg to print out every event number #425 (comment)