new create_bonds command

FYI, we added a create_bonds command in the 21Jan15 patch,
which can be used to auto-generate bonds (without a data file)

for simple systems, e.g. between near neighbors in a lattice
of atoms. The Movies page has a small movie that uses
it in the “dynamic load-balancing” section to keep an agglomeration
of particles together when they hit a wall.