Changes to ChCollisionModel method prototypes

I have made some fixes to the Add***() virtual methods in ChCollisionModel to pass the position vector and rotation matrix through const refs instead of pointers.

This simplifies and cleans up the code and removes the reliance on nonstandard compiler extensions (i.e., it fixes the 'taking address of temporary' linker warnings with gcc or warning C4238 with VS: 'class rvalue used as lvalue').

I have tested this both on Windows (VS2010) and Linux (gcc).  Note that some updates would have to be made to the derived class ChModelSphereSet when chrono-mpi is reinstated.