Add semi-implicit Euler integrator

[diegoferigo]

This PR implements a semi-implicit Euler integrator as described in https://en.wikipedia.org/wiki/Semi-implicit_Euler_method.

Contrarily to the other integrators we have (forward Euler, Runge-Kutta 4), the new integrator does not work on any generic PyTree. In fact, this integrator is specific to Hamiltonian systems where the ODE can be described in terms of positions and velocities.

As an overview, our ODE is structured as follows:

• State $x(t)$: ODEState
• State derivative $\dot{x}(t) = f(x(t), u(t))$: ode.dx_dt
• Other integrators:
  • Forward Euler
  • RK4
• Model integration: high_level.model.Model.integrate