add option to compute chunk/atom to access the number of chunks as a global scalar

**Output info:**

This compute calculates a per-atom vector, which can be accessed by

any command that uses per-atom values from a compute as input.  See

the :doc:`Howto output <Howto_output>` doc page for an overview of

This compute calculates a per-atom vector (the chunk ID), which can

be accessed by any command that uses per-atom values from a compute

as input.  It also calculates a global scalar (the number of chunks),

which can be similarly accessed everywhere outside of a per-atom context.

See the :doc:`Howto output <Howto_output>` doc page for an overview of

LAMMPS output options.

The per-atom vector values are unitless chunk IDs, ranging from 1 to

*Nchunk* (inclusive) for atoms assigned to chunks, and 0 for atoms not

belonging to a chunk.

belonging to a chunk.  The scalar contains the value of *Nchunk*.

Restrictions

""""""""""""

  if (narg < 4) error->all(FLERR,"Illegal compute chunk/atom command");

  peratom_flag = 1;

  scalar_flag = 1;

  extscalar = 0;

  size_peratom_cols = 0;

  create_attribute = 1;

  for (int i = 0; i < nlocal; i++) chunk[i] = ichunk[i];

}

/* ----------------------------------------------------------------------

   to return the number of chunks, we first need to make certain

   that compute_peratom() has been called.

------------------------------------------------------------------------- */

double ComputeChunkAtom::compute_scalar()

{

  if (invoked_peratom != update->ntimestep)

    compute_peratom();

  invoked_scalar = update->ntimestep;

  return (scalar = nchunk);

}

/* ----------------------------------------------------------------------

   set lock, so that nchunk will not change from startstep to stopstep

   called by fix for duration of time it requires lock

  void init();

  void setup();

  void compute_peratom();

  double compute_scalar();

  void set_arrays(int);

  double memory_usage();