[Halld-offline] DVertex from MC

[Paul Mattione]

I've added (and checked into svn) a new parameter to the libraries/PID/DVertex factory called "PID:VERTEX_FROM_MC."  Setting this parameter to 1 from the command line will cause the factory to generate only one DVertex, set it to the first thrown MC vertex (regardless of how many vertices were thrown within a given event), set the vertex time to zero, and assign all of the DChargedTracks to it.  This parameter is set to 0 by default (disabling this functionality).  

 - Paul