Add the Pendulum as an exercise in Simulation

[boisgera]

Demonstrate basic solve_ivp call then dense_output for $\theta(0) = \pi/2$ and $\dot{\theta} = 0$, then do the same thing for $\theta(0) = 0$ and $\dot{\theta}=2 \sqrt{g/l}$ and study the influence of atol and rtol.