Particles_advance(real(in) :: dtOld, real(in) :: dtNew)
Time advancement routine for the Particle Unit, called from within a single block. Particle velocities are interpolated from the grid. The two different time steps are needed if a multi-step method changes timestep sizes between steps. (e.g. Predictor-Corrector or Midpoint) At the end of moving particles internally within a block, Particles_advance calls the routine Grid_moveParticles, which redistributes particles that have gone outside the current block.
dtOld: real time step for the last velocity scheme (not used in some routines) dtNew: real time step for the current round of sweeps
This routine is called TWICE within each time step. It is called once after each split-direction Hydro sweep.