add crude check to print warning when using compute cnp/atom on multi-type system

  if (cutoff < 0.0) error->all(FLERR,"Illegal compute cnp/atom command");

  cutsq = cutoff*cutoff;

  // apply check for single type atoms in compute group

  int lasttype = -1;

  int n = -1;

  for (int i=0; i < atom->nlocal; ++i) {

    if (atom->mask[i] & groupbit) {

      if (lasttype != atom->type[i]) {

        lasttype = atom->type[i];

        ++n;

      }

    }

  }

  int all_n = 0;

  MPI_Allreduce(&n,&all_n,1,MPI_INT,MPI_MAX,world);

  if (all_n > 0)

    error->warning(FLERR,"Compute cnp/atom requested on multi-type system");

  nmax = 0;

}