Closing a SyncingVOFile now automatically flushes it first.
Changed open method of WorkingContext to create a SyncingVOFile if 
we are in a VOSpaceWorkingContext and a normal file if we are in a 
LocalDirectoryWorkingContext.
Added integration tests for using SyncingVOFile with MPCWriter and 
StreamingAstromWriter.  Added no-op seek method to SyncingVOFile since
it is used in the StreamingAstromWriter to read from the beginning of the
file (the default behaviour for SyncingVOFile).
Removed mode parameter from context.open calls in 
LocalProgressManager since this is no longer a valid parameter.
Added another integration test for storage to make sure VOSpace files
are not modified by read only operations.