Added code to create pressure compute snap_press behind the scenes

# set up new-style global compute

compute 	snap all snap ${rcutfac} ${rfac0} ${twojmax} ${radelem1} ${radelem2} ${wj1} ${wj2} rmin0 ${rmin0} quadraticflag 0 bzeroflag 0 switchflag 0

compute 	snap_press all pressure NULL virial

fix 		snap all ave/time 1 1 1 c_snap[*] file compute.snap.dat mode vector

thermo 		100

  // 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";

  newarg[2] = (char *) "pressure";

  newarg[3] = (char *) "NULL";

  newarg[4] = (char *) "virial";

  modify->add_compute(5,newarg);

  delete [] newarg;

  int ivirial = modify->find_compute(id_virial);

  if (ivirial == -1)

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

  c_virial = modify->compute[ivirial];

}