Add a parameter sweep mode

[laprej]

It would be awesome if, given a model, we were able to find optimal batch and gvt interval.

[JohnPJenkins]

Is ROSS reentrant?

for (batch = 1; batch < 16; batch++) {
  g_tw_batch = batch;
  for (interval = 1; interval < 16; interval++) {
    g_tw_gvt_interval = interval;
    MPI_Barrier();
    clock_gettime(start);
    tw_run();
    clock_gettime(end);
  }
}