       (3) z velocity component (velocity units)

initialize velocities of atoms to a specified temperature
use appropriate number of coeffs for a particular style
cannot be done before a data or restart file is read
by default, velocities are created for all atoms - this can be overridden
  by first using a "create group" command
for uniform and Gaussian styles velocities are created in 
  processor-independent fashion - is slower but gives the same initial 
  state independent of # of processors
for unifo