Implemented Inheritance for Particles

- All particles now inherit from a base class, "P0_Basic_Particle".
-  Compound classes inherit from "P0_Compound_Particle" (which in turn
inherits from basic).

This cuts down on code bloat: the base class declares all variables that
are common between inheriting classes.  The compound class also declares
some methods that all compound particles use.

Check out the "particle_zone" level.