Simplify integrators module structure

[flferretti]

This PR has the aim of simplifying the integrators module. This will ease up the process of adding new types of integrators and facilitate the debugging.

In order to do this, I made the following changes:

• the class IntegratorType has been moved from ode_integration to integrators. This has no effect on the rest of the code as the integrators module is imported before ode_integration
• the odeint_INTEGRATORTYPE functions have been concentrated in one
• the odeint_INTEGRATORTYPE_one_step functions have been merged in one oneint_one_step that changes the body function only. For the manifold integrator, the additional operation have been moved inside a conditional

Since @diegoferigo is working on the variable step integrators, we could think to merge this PR after rebasing on the #72

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📚 Documentation preview 📚: https://jaxsim--82.org.readthedocs.build//82/

[diegoferigo]

I was doing the review in the online VSCode and it got sent before actually finishing it :)

It looks good even though there's some issue with typing. In fact, the Euler/RK integrators are generic and can work on any pytree. Instead, the semi-implicit variants and those that integrate the quaternion on SO(3) expect and return an ODEState.

I originally developed these integrators for a possible new external library, but it always made sense to keep them inside the project. Eventually, the schemes that work better are those with the base orientation on the manifold, and they are pretty custom for floating-base robots. I have a better implementation of schemes based on Butcher tableau in #72, if we feel like, we can use those resources also for the vanilla fixed-step integrators (forward euler, RK4).

[diegoferigo]

Looks good! Some remaining minor comments.