Track discovery triplet first and no auto discovery for track writer

The discovery triplet is now transplanted to the front of the track workunit (closes #229).

Also added auto_discovery flag to MPCWriter.  By default it is True, in which case it has the behaviour required for reals processing as added in #236.  The MPCWriter used in the TracksWorkUnit sets it to False so that no additional discovery asterisks are generated during tracks processing.  This should address #237.