Always use the link frame in body-fixed and mixed output representations of model Jacobian

[diegoferigo]

This PR superseeds #164, in view of the discussion started in #164 (comment).

After this PR:

• Calling jaxsim.api.model.generalized_free_floating_jacobian in body-fixed and mixed output represenations always return the stacked ${}^L J_{W,L}$ and ${}^{L[W]} J_{W,L}$, respectively.
• Since now there is some additional logic to compute the pose of the link, the function runs a bit slower than before (and cannot be worked around). This method was originally enhanced in Optimize Jacobian algorithm #121.
• I could have refactored jaxsim.api.link.jacobian to call jaxsim.api.model.generalized_free_floating_jacobian and extract the i-th Jacobian, but the current implementation is much faster and I believe that having the logic partially duplicated might make sense in this case.
• The references helper, by default, converts the forces in the correct frame that matches the output representation of the Jacobian so that $J^\top \mathbf{f}$ is correct. This was correct also before, but it was passing through the $B$ frame instead of the $L$ frame.

[flferretti]

LGTM, thanks!