simplify compute creation

  // find compute for reference energy

  std::string id_pe = std::string("thermo_pe");

  int ipe = modify->find_compute(id_pe);

  int ipe = modify->find_compute("thermo_pe");

  if (ipe == -1)

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

  c_pe = modify->compute[ipe];

  // add compute for reference virial tensor

  std::string id_virial = std::string("snap_press");

  std::string pcmd = id_virial + " all pressure NULL virial";

  modify->add_compute(pcmd);

  modify->add_compute("snap_press all pressure NULL virial");

  int ivirial = modify->find_compute(id_virial);

  int ivirial = modify->find_compute("snap_press");

  if (ivirial == -1)

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

  c_virial = modify->compute[ivirial];