Fix jaxsim.api.model.link_contact_forces

[xela-95]

In this PR I added to the jaxsim.api.model.link_contact_forces API the external link forces and joint torques as additional input arguments. These are then passed to jaxsim.api.contact.collidable_point_forces. In this way, the contact forces computed with the rigid contact model (but this applies also to relaxed-rigid ones I think) will consider all the forces acting on the model.

I faced an issue when I was performing simulations with @flferretti using rigid contacts, and we noted that the contact forces given by jaxsim.api.model.link_contact_forces were only related to the weight of the model and were not considering other forces potentially acting on the model.

The computation of the contact forces used inside model.step was already correct: through ode.system_velocity_dynamics it calls contact.collidable_point_dynamics with the correct arguments, while contact.collidable_point_forces was not aligned.

Let me know what do you think of this :)

---

📚 Documentation preview 📚: https://jaxsim--242.org.readthedocs.build//242/

[xela-95]

Also I'm not sure where it is more appropriate to validate the input arguments in the chain of function calls, for now I just check the input link forces and joint torques in jaxsim.api.model.link_contact_forces.

[flferretti]

Thanks a lot @xela-95 for the fix, LGTM

[diegoferigo]

Thanks @xela-95, LGTM!