make native dump reader compatible with dump files containing ITEM: UNITS

  char *eof = fgets(line,MAXLINE,fp);

  if (eof == NULL) return 1;

  // skip over unit information, if present.

  if (strstr(line,"ITEM: UNITS") == line)

    read_lines(2);

  if (strstr(line,"ITEM: TIMESTEP") != line)

    error->one(FLERR,"Dump file is incorrectly formatted");

  read_lines(1);

  int rv = sscanf(line,BIGINT_FORMAT,&ntimestep);

  if (rv != 1)

    error->one(FLERR,"Dump file is incorrectly formatted");

  return 0;

}