particle-swarm: Remove unused "position" property
particle-swarm: Add a global acceleration property

This acceleration vector is applied to all particles uniformly, and can
be used to model physical properties such as wind or gravity.
boids: Change demo to white-on-black theme
particle-swarm: Spawn particles around perimeter
particle-swarm: Enforce speed limit once
particle-swarm: Yes, this is really working
particle-warm: Add a speed property to particles
particle-swarm: Refactor particle update func names
particle-swarm: Merge particle and boundary avoidance funcs
particle-swarm: Add a comment
particle-swarm: Refactor rule velocities

This replaces the individual velocity vectors for each rule with a
single change in velocity vector.
particle-swarm: Model swarms in three dimensions