[ODE] Modelling Joint Friction

[Erin Catto]

Well, all you need from ODE is the constraint force imposed by the motor.
You can apply the position penalty in your own stepping function. So any
change to ODE would be minimal, just an accessor.

I haven't implemented this technique for friction, but I have used penalties
for motor positioning. It works quite well. Please let us know if it works
well for static friction.

Erin