add message about -DLAMMPS_BIGBIG incompatibility of scatter/gather functions

void lammps_scatter_atoms(void *, char *, int, int, void *)

void lammps_scatter_atoms_subset(void *, char *, int, int, int, int *, void *) :pre

void lammps_create_atoms(void *, int, tagint *, int *, double *, double *,

                         imageint *, int) :pre

The gather functions collect peratom info of the requested type (atom

coords, atom types, forces, etc) from all processors, and returns the

same vector of values to each calling processor.  The scatter

passed by all calling processors, to individual atoms, which may be

owned by different processors.

IMPORTANT NOTE: These functions are not compatible with the

-DLAMMPS_BIGBIG setting when compiling LAMMPS.  Dummy functions

that result in an error message and abort will be subsituted

instead of resulting in random crashes and memory corruption.

The lammps_gather_atoms() function does this for all N atoms in the

system, ordered by atom ID, from 1 to N.  The

lammps_gather_atoms_concat() function does it for all N atoms, but

lammps_scatter_atoms_subset() function takes a subset of IDs as an

argument and only scatters those values to the owning atoms.

void lammps_create_atoms(void *, int, tagint *, int *, double *, double *,

                         imageint *, int) :pre

The lammps_create_atoms() function takes a list of N atoms as input

with atom types and coords (required), an optionally atom IDs and

velocities and image flags.  It uses the coords of each atom to assign