Pool vectors for photons to improve performance

[jbphet]

While working on phet-io state, @zepumph suggested that photon movement and state setting might be more efficient if we use pooling for the vectors.

There are apparently some good examples in Projectile Motion, search for Vector2.pool.create( to see them.

[zepumph]

Yes this seems like it would be pretty dang easy to implement inside of Photon.ts, and would give us some gains. Over in #301 we didn't see a bunch of performance problems when setting a couple thousand photons to the state (<500ms on average), but if this is simple, since we KNOW just how many objects are being used and discarded, it seems worth while. I would have it on a lowish priority, and its potential simplicity would up it a medium priority.