Migrate jaxsim.api package away from PhysicsModel

[diegoferigo]

• Remove the usage of PhysicsModel in favor of the combination of JaxSimModel and the new KinDynParameters.
• Move RBDAs to jaxsim.rbda.
• Add the standard gravity constant as additional input of the RBDAs using it.
• Move terrains to jaxsim.terrain.
• Move ODE{State|Input}, PhysicsModel{State|Input}, SoftContactsState to jaxsim.api.ode_data.
• Move VelRepr to jaxsim.api.common.
• Remove jaxsim.sixd pass-through in favor of plain jaxlie.
• Update RBDAs in jaxsim.rbda to use JaxSimModel instead of PhysicsModel.
• Update argument names of RBDAs.

The unused classes will be removed in another PR. In this one, the aim is to make everything work with the new KinDynParameters class introduced in #101.

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📚 Documentation preview 📚: https://jaxsim--112.org.readthedocs.build//112/

[diegoferigo]

Waiting @flferretti to be back, I added @traversaro as reviewer. This PR is not merging into main, we can do a final round of review in #108.

[flferretti]

LGTM, thanks!

[diegoferigo]

I just realized that the removal of PhysicsModel (JaxSimModel.physics_model) might have solved #103 also for models with joints @traversaro @flferretti.

[traversaro]

I just realized that the removal of PhysicsModel (JaxSimModel.physics_model) might have solved #103 also for models with joints @traversaro @flferretti.

Great!