Improve handling of multiple JEventSources

[nathanwbrei]

Previously, multiple event sources would each be assigned separate JEventSourceArrows and be run at the same time. This PR changes JEventProcessorArrow so that it accepts a vector of JEventSources and runs them one after another until each completes or reaches the max jana:nevents'. This leads to fewer files/sockets being open simultaneously, better performance due to fewer calls to JFactory::ChangeRun` etc, less work for the scheduler, and a more easy-to-reason-about event stream. The old behavior is easy to bring back in the JTopologyBuilder if we ever want it.

This addresses issue #146.

It includes some refactorings:

• JEventSource::DoNext now handles all of the logic for setting up a JEvent prior to JEventSource::GetEvent. Previously some of this was mixed in with JEventSourceArrow.
• JEventSource::DoNext() now calls JEventSource::Close() when JEventSource::GetEvent() declares itself finished.
• JEventSource::SetRange() is replaced with JEventSource::SetNEvents and SetNSkip. The corresponding getters have been added.

There is one small behavior modification:

• JEventSource::Open() is now always called from JEventSourceArrow::execute(), whereas before it was called from JEventSourceArrow::initialize(). This changes the behavior of JApplication::Quit() when called from inside JEventSource::Open, as well as Ctrl-C: Previously, no events would be emitted and Run() would return before even firing up the workers. Now, Quit() will trigger a graceful shutdown but won't immediately stop the source from emitting JEvents. This is less useful but more consistent and correct: if you want to handle an error condition in JEventSource::Open(), you should throw a JException instead.

[faustus123]

Do I understand correctly that this will not call finalize on any JEventSource until the end of processing? I guess more to the point, will the source files all be kept open until the end of the program vs. closing them earlier? I ask since we have seen where people may run jan giving it a hundred or more small skim files in one job and running into issues with having that many open file descriptors.

[nathanwbrei]

@faustus123 Nope, JEventSources are closed as soon as possible, in other words, in DoNext(). The only time that a JEventSource gets closed during finalize() is when the user Ctrl-C'ed or called JApplication::Quit().