Adding unit conversions to Physics.h

I've updated the Physics.h to include a series of common unit conversions used throughout the code.

So far I've included:

• m <--> mm
• s <--> ns
• T <--> kG
• V <--> kV
• GeV/c^2 <--> kg

I've then used these conversions to replace the hard coded conversions in the following files:

• ParallelCyclotronTracker.cpp
• RK4.h
• OpalMultipoleT.cpp

I've linked this issue. There's a bit more discussion on this in the comments there.