Suggestion for new library function lammps_scatter_atoms

The library function lammps_scatter_atoms will fail (actually, it will
probably cause a segmentation error) if an atom map has not been defined
prior to its use. I recommend an error (or at least a warning) message
be issued if one attempts to use the procedure without an atom map.

Several of the methods in library.cpp might benefit from sending error
messages instead of just returing without doing anything---the rest of
LAMMPS will give such errors (and abort the calling program) through
the library interface, so in that sense I don't think it's any more
obtrusive to the user.

Karl D. Hammond
University of Tennessee, Knoxville
[email protected]...

just added a warning - will be in next patch.

The lib philosophy here is that it's the caller's responsibility
to check error returns (e.g. a NULL), or pay attention to the warning message
in this case.