[Halld-physics] Resonances No Longer Allowed in DReactionStep

[Paul Mattione]

I've just checked in a change to the svn repository that prevents users from setting a resonance PID in their DReactionStep objects.  Setting "Unknown" is also no longer allowed, except it can be set for a missing final state particle.  For example, if you are studying the channel:

g, p -> pi+, pi+, pi-, (n)

you can no longer setup your DReaction with an intermediate rho resonance, e.g.:

g, p -> pi+, rho, (n)      rho -> pi+, pi-

This is because inclusion of resonances (such as above) can create false additional particle combinations, since it is not correct to assign a pion as exclusively belonging to the rho decay.  

If your analysis plugin contains a resonance, you'll have to remove it or else the program will exit.  

 - Paul