[Halld-offline] PID Software Changes

[Paul Mattione]

As discussed in today's offline meeting, I've modified the DNeutralParticleHypothesis code (and checked it into SVN) so that it only generates neutral particles using the center of the target as the vertex, as opposed to using the vertices reconstructed from the charged tracks.  

Note that since one DParticleSet object is created for each DVertex, I set it so that each DNeutralParticle object is added to each DParticleSet.  

 - Paul