reset Modify::n_timeflag in post_run() so we won't skip computes defined between runs

void Modify::post_run()

{

  for (int i = 0; i < nfix; i++) fix[i]->post_run();

  // must reset this to its default value, since computes may be added

  // or removed between runs and with this change we will redirect any

  // calls to addstep_compute() to addstep_compute_all() instead.

  n_timeflag = -1;

}

/* ----------------------------------------------------------------------