Fix JaxSimModelReferences.apply_frame_forces

[flferretti]

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📚 Documentation preview 📚: https://jaxsim--220.org.readthedocs.build//220/

[diegoferigo]

LGTM! It seems to me that before there were two problems:

1. The returned references was wrongly switched to Inertial instead of Body when the link forces were applied (and the name of the variable W_f_L_i should have been L_f_L_i).
2. The returned references had a different velocity representation than self.

Did I get it right?

[flferretti]

1. The returned references was wrongly switched to Inertial instead of Body when the link forces were applied (and the name of the variable W_f_L_i should have been L_f_L_i).

I guess it's the other way around, I was converting the frame forces to body and few lines below I was summing them using Inertial representation. Keeping everything in inertial, made more sense at this point.

2. The returned references had a different velocity representation than self.

This is correct!

There was also a third mistake that had to do with the name of a variable:

- frame_idx=frame_idxs
+ frame_index=frame_idxs

[diegoferigo]

I guess it's the other way around, I was converting the frame forces to body and few lines below I was summing them using Inertial representation. Keeping everything in inertial, made more sense at this point.

Yep what I meant is that the previous logic to express them in the link frames L was ok, the problem with that approach is that you were applying them as they were Inertial instead of Body. Anyway, I agree that keeping everything in inertial as after this PR is more clean.