Be smart about updating flock center and average velocity

[mlms13]

Currently, the flock center point and average velocity are updated at the beginning of each positionBoids cycle. This requires a completely separate iteration over each boid in the flock. What if we lived in a world where we updated the flock's center/velocity each time we added or re-positioned a boid? This way the values would always be up-to-date, and we could do everything in a single loop, instead of two.

Maybe we want to do some FPS profiling... This update would mean setting those values more often, but it would also mean dropping one of our successive iterations. I guess it depends a little how much overhead there is in iteration vs re-writing properties.