[Halld-offline] Particle Class Structure

[Paul Mattione]

I think that the current class structure has become a little  
fragmented at the top level, and I'd like to suggest a few changes to  
make things simpler and more intuitive.  In addition to DChargedTrack  
and DPhoton no longer being used to create the DPhysicsEvent class,  
the reconstruction algorithm assumes that every calorimeter shower  
comes from either a charged particle or a photon, excluding neutrons  
(such as from Lambda decay).

Here's a link to an overview of the current top-level class structure,  
and a proposal for a new (slightly different) class structure that I'm  
working on:

http://www.jlab.org/Hall-D/software/wiki/index.php/Mattione_Particle_Classes

The next offline meeting is over a week away, but in the meantime I  
would appreciate it if you guys could let me know what you think.   
Thanks!

  - Paul Mattione