Return ISynchronize when secondary file is to be opened

In the multithreaded framework, a synchronization point is needed for the one file two file feature before a secondary input file is opened asynchronously with the opening of primary files.  This pull request provides this synchronization.

This pull request also causes getNextItemType() to read the EventAuxiliary when the next item is an event.  Before this pull request, the EventAuxiliary was not read until readEvent() time.