#5 Verlet and removing CM velocity

[J. Matthew D. Lane]

The verlet procedure used to remove the center of mass
velocity from the dynmaics. In order to allow for
shocks, this was disabled. There should be an
intelligent switch for when to use this feature and
when not to. By intelligent, I mean that zeroing or
non-zeroing the CM velocity should depend on a param
that is set based on the type of simulation run (e.g.
crack or shock)