Dynamic load balancing

A 2d circle of particles is placed in a larger box with Lennard-Jones walls and given an initial velocity so it bounces off them. In the first animation it disintegrates; in the second and third, bonds added with create_bonds hold it together at slow and fast impact speeds. Because the particle density varies strongly in time, fix balance rcb continuously rebalances the particles across the 8 processors — the processor sub-domains are drawn and particles are colored by which processor owns them.