[ODE] Collision impact force

[Tyler Streeter]

> Excuse my ignorance, but does contact depth really show the force of the impact, or just what it happened to be the relative position of the body at that point of time with that specific time step?
> 

It's sort of a crude measurement (compared to the joint feedback way),
but it still gives you some idea of the impact magnitude when two
objects collide.  Since ODE continuously tries to force them apart
when they intersect, a larger penetration depth implies that the
impact force was great enough to overpower ODE's restitution forces.

Tyler