Step the simulator over horizon

[diegoferigo]

This PR adds the possibility to step the simulator over a time horizon instead of being stepped over only a single $\Delta t$.

Two modalities are supported:

1. Pure open loop stepping. All the simulated models are integrated over the horizon in open-loop, i.e. their input (joint torques and link external forces) could either be either kept constant or just applied in the first step.
2. Closed loop stepping. In this case, this PR introduces experimental callbacks that, for example, could include logic of robot controllers. The callback object could be a generic PyTree and could have an internal state.

Callbacks could be useful in any case to extract data from the entire horizon for logging purpose.

Note that this kind of integration over horizon is mostly beneficial when it is combined with jax.jit. In this way, the simulator can maximize runtime performance by compiling together a possibly long forward simulation.

Note: We do not support integrating in parallel multiple cloned models belonging to a single simulation instance. Integration parallelism is performed by creating multiple simulator.JaxSim objects with only one model. The overhead of the simulator instance is almost negligible, and this approach makes calling jax.vmap explicit from the user point-of-view.

Note: The new JaxSim.step_over_horizon method, contrarily to all the other methods that can be either called in an object-oriented or a functional way thanks to JaxsimDataclass, is purely functional. This is due to the logic related to the callbacks.