[lammps-users] dump sorts

FYI, I added an option (19 Sept patch) to
allow the dump_modify sort command to be
used with any dump style. So you can now
have dump snapshots sorted by atom ID
or any other column, if you enable it.
Not that I recommend it, but some users
seem to want it.

This involved changes (actually simplifications)
to all the dump styles, and
I can't test all them easily, so
let me know if there are problems.