TracksParser now makes a note on the reading which is the discovery
reading by setting a new discovery attribute to True.  The TracksWorkUnit
then looks for this reading and moves it and the next two to the front
of the reading list.  This results in the discovery triplet being at the front 
of the track work unit.
Merge branch 'master' into track_discovery_first
Added auto_discovery flag to MPCWriter to control whether the 
discovery asterisk is automatically added to the first observation flushed
or not.  Reals has it set to True, track has it set to False.
Merge pull request #240 from drusk/track_discovery_first

Closes #229, #236, #237.