TrackingTruthProducer set vertex fix

Fix to the old TrackingParticle producer so that the vertex position is set properly.  This fixes the discrepancy in the dz resolution between the old and new TrackingParticle implementations.  Only file changed is SimGeneral/TrackingAnalysis/plugins/TrackingTruthProducer.cc, don't know why the diff says 5851 files have changed.  Also, the file is deprecated anyway so don't know if it's worth putting the change in.