Loading MJCF descriptions in Pinocchio

[stephane-caron]

This feature request was proposed by @lvjonok here:

Since Pinocchio v3.0.0, it's possible to use MJCF to create models. However, the Pinocchio loader in robot_descriptions currently utilizes the buildFromURDF method.

Do you think this could be better addressed using a variant keyword, or should we handle it with an if-statement to check whether module.MJCF_PATH or module.URDF_PATH is available?

[stephane-caron]

This feature has been added in #101: if your Pinocchio version supports it, the loader will be able to load MJCF descriptions.