ump all atom positions to a file every this many timesteps
  (every this many iteration when the minimizer is invoked)
when rRESPA is enabled, this is steps of outermost loop (longest timesteps)
positions are also dumped at the start and end of each run
when dumped during minimization, all dumps will have the same timestamp
  since the timestep does not change during minimization
value of 0 means never dump
any previous file is closed
new filename can exist, will be overwritten
atom positions in dump file a