optimize neighbors function

[MollsReis]

performance of this function is limiting boid swarm size

[MollsReis]

• can get this to n! if we loop over once and each boid shares the distance with the boid measured against
• investigate http://en.wikipedia.org/wiki/Z-order_curve as a possibility

[MollsReis]

87000f0 helps a bit but this can be further improved