This example simulates the motion of double pendulum.

It uses method of Runge-kutta. Type "simulation --help" to get more
help on running the code.

By default, it uses fixed-step Runge-Kutta method with relatively
small  step 0.003.

One can choose adaptive step by adding option "-adaptive".

Plotting the old path slows down simulation. To remove this feature, use
option "-nopath".

The simuation should preserve energy of the pendulum. Due to numerical inaccuracy,
this will not be the case for large energy and large time-step.

The adaptive RK-method seems to perform superior and conserves energy for very long times.

You can change energy by the option "-E double"