Remove object references cycle in LinkDescription

[younik]

Currently, LinkDescription has the attributes parent and children that are LinkDescription. This creates a reference where a LinkDescription references its parent, which references back to the LinkDescription.

This can create infinite recursion in case we call a function on the attributes of LinkDescription recursively, for example in __repr__, __hash__ or dataclasses.asdict.

The last one is needed to implement a Gymnax environment, which I believe is a common use case.

In this PR, I changed the parent attribute to be the parent's name, instead of the object itself. This doesn't create a problem since most of the time we only need the name and there is a dictionary mapping names to LinkDescription.

To reproduce the issue:

from dataclasses import asdict
import jaxsim.api as js

model = js.model.JaxSimModel.build_from_model_description(
    model_description=...,
    is_urdf=True,
)
asdict(model)

---

📚 Documentation preview 📚: https://jaxsim--374.org.readthedocs.build//374/

[flferretti]

Thanks a lot @younik, this is very useful also for #351 and for @xela-95! I just added a couple minor comments

[xela-95]

Thanks! LGTM!