RK45 needed for acrobot/pendubot

[jmichaux]

It looks like there is a need to implement runge-kutta 45 for the acrobot/pendubot. I've noticed 3 things:

1. The solution will diverge from the fixed point, even in the absence of torque commands or perturbations.

2. From some initial states the pendula appear to gain kinetic energy.

3. From some initial states the dynamics seem to dampen, as if there were friction in the system.