remove pointless stringstream abuse

  if (narg < 0) error->all(FLERR,"Illegal third_order command");

  int iarg = 0;

  const char *filename = "third_order.dat";

  std::stringstream fss;

  while (iarg < narg) {

    if (strcmp(arg[iarg],"file") == 0) {

      if (iarg+2 > narg) error->all(FLERR, "Illegal third_order command");

      fss << arg[iarg + 1];

      filename = fss.str().c_str();

      filename = arg[iarg + 1];

      file_flag = 1;

      iarg += 2;

    } else if (strcmp(arg[iarg],"binary") == 0) {