Add frame example

[CarlottaSartore]

This PR adds the example for computing velocity and transformation of the frames. In addition, it introduces minor changes to align the frames API with the link's one

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📚 Documentation preview 📚: https://jaxsim--300.org.readthedocs.build//300/

[flferretti]

Thanks a lot @CarlottaSartore! LGTM. I just added a minor comment

[xela-95]

Thanks @CarlottaSartore for your contribution! I just left some minor comments.

Other than this, would you add a little unit test on the jaxsim.api.frame.velocity function to avoid regressions in future? It can be added to https://github.com/ami-iit/jaxsim/blob/main/tests/test_api_frame.py

[CarlottaSartore]

Thanks @CarlottaSartore for your contribution! I just left some minor comments.

Other than this, would you add a little unit test on the jaxsim.api.frame.velocity function to avoid regressions in future? It can be added to https://github.com/ami-iit/jaxsim/blob/main/tests/test_api_frame.py

Thanks @xela-95, I have implemented the CI in 5fdbe5b

Let me know if there ar any additional changes needed !

[CarlottaSartore]

The tests are not passing, checking why

[CarlottaSartore]

I shoudl have fix with commit 436fde6

The error was that Jaxsim creates a single vector organise as : $[l_0, ..., l_{nl}, f_0,..., f_{nf}]$
What happens is that the idx returned by frame name_to_idx is the idx inside such vector and not the index of the vector containing only frames.
Thus such index should be $>nl$ and $<nl+nf$

[xela-95]

The error was that Jaxsim creates a single vector organise as : [ l 0 , . . . , l n l , f 0 , . . . , f n f ] What happens is that the idx returned by frame name_to_idx is the idx inside such vector and not the index of the vector containing only frames. Thus such index should be > n l and < n l + n f

Right nice catch!

[flferretti]

Minor typo

[xela-95]

Now it's good for me! :)

[CarlottaSartore]

Thanks @xela-95, and @flferretti I should have also implemented your review, is it right ? can I proceed with the merge ?

[flferretti]

Thanks @CarlottaSartore, LGTM