Remove absolute time from JaxSimModelData and introduce JaxSimModel.time_step

[diegoferigo]

In this PR, I'm finally addressing a longstanding problem that complicated the usage of 32-bit precision as described in #136. After having explored more complex solutions, we decided to stop tracking the absolute simulation time within JaxSim, moving the responsibility to downstream code if necessary.

More details:

• Remove JaxSimData.time() and JaxSimData.time_ns.
• Add a new JaxSimModel.time_step that stores the default integration $\Delta t$. This can be useful in some particular cases, for example those system dynamics that need to know the $\Delta t$ used by the integrator (e.g. semi-implicit integrators, visco-elastic contacts, etc).
• Add a new t0 argument to jaxsim.api.model.step, so that it's still possible to run AD w.r.t. the time if the dynamics considered by the integrator is time-dependent (this is rarely the case).

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📚 Documentation preview 📚: https://jaxsim--262.org.readthedocs.build//262/

[flferretti]

Thanks a lot Diego! This was very much needed! I just left a couple comments