particle-swarm: Add (partile|velocity)_sum optimisations

This reduces the time spent performing summing operations from O(n) to
O(1).
particle-sum: Fix typo in velocity optimisation
particle-swarm: Minor refactor
particle-swarm: Create particles within barrier
boids: Configuration tweaks
boids: Change color scheme
boids: Configuration tweaks
particle-swarm: Add a boundary_threshold property