Add support for continuous joints

[diegoferigo]

The support in ROD to build programmatically and parse models with continuous joints has been added in ami-iit/rod#40.

A continuous joint, contrarily to revolute joints, does not have /joint/axis/limit/lower|/joint/axis/limit/upper. The new version of ROD allows to build a trivial Limit object with all attributes set as None. In this case, we set the upper and lower limits to the maximum/minimum floating point number in the active precision of JAX (either 64 bits or 32 bits).

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📚 Documentation preview 📚: https://jaxsim--228.org.readthedocs.build//228/

[flferretti]

Thanks Diego, LGTM! I just have a curiosity, should these changes be propagated in JointType?

jaxsim/src/jaxsim/parsers/descriptions/joint.py

Lines 15 to 20 in 58b803c

	@dataclasses.dataclass(frozen=True)
	class JointType:
	Fixed: ClassVar[int] = 0
	Revolute: ClassVar[int] = 1
	Prismatic: ClassVar[int] = 2

[diegoferigo]

Great question! I don't think we need to do that. The logic of this PR just introduces the support of parsing continous joints (supported in SDF). The difference between a continuous joint and a revolute joint is that continuous joints do not have position limits.

This PR basically parses continuous joints and treats them as revolute joints by setting their position limits as the maximum float in JAX precision.

[flferretti]

Alright, thanks for the explanation!