Runge-Kutta 4: Logic enabled

4th order Runge-Kutta implementation that allows user to process complex calculations each timestep

This RK4 implementation allows the user to program any complicated math and logic into the timesteps by using a function-call to get the system of equations to be solved. For example, you can simulate a valve that opens at t=5 using an "if" statement in your system of equations that switches the valve equation from the closed to the open state at t=5.
A video tutorial of the code is available at http://youtu.be/ps7ixASlSgc