Particle.*: Rewrote setV to setVel --- fits rest of names better.
System.*: Added open / close / save methods.

Also added counter for time and added start-time to constructor
argument.
System.cpp: Forgot @param doc for t0 in constructor.
System.hpp: Permission fixes for subclasses.
SystemRK4.cpp: Copied code from Prj03 and fadjusted to utilize new structure.