Added code to create pressure compute snap_press behind the scenes

  // add compute for reference virial tensor

  char *id_virial = (char *) "snap_press";

  char **newarg = new char*[5];

  newarg[0] = id_virial;

  newarg[1] = (char *) "all";

  int ivirial = modify->find_compute(id_virial);

  if (ivirial == -1)

    error->all(FLERR,"compute snap requires that compute snap_press exists.");

    error->all(FLERR,"compute snap_press does not exist.");

  c_virial = modify->compute[ivirial];

}