Added runge kutta 4 ODE integrator.
Added runge kutta 4 ODE integrator.

We can quibble whether this belongs here, and it probably doesn't,
but doing runge kutta with scipy is pretty painful.

Censio/filterpy,BrianGasberg/filterpy,BrianGasberg/filterpy,rlabbe/filterpy,aitatanit/filterpy,aitatanit/filterpy,Censio/filterpy,rlabbe/filterpy