Load scenes rather than robot files for MJCF descriptions

[stephane-caron]

When loading a robot description from MuJoCo Menagerie, it seens it is more common to load the scene.xml files for each description rather than the individual robot files (that have, notably, no ground).

The proposal is then to make all MJCF descriptions loaded via robot_descriptions go for scene.xml by default, with optionally a variant #62 for bare robots.

[stephane-caron]

@kevinzakka Do you think this would make sense for the majority of users?

I can take care of making a PR for this, sometime in a not so-long time.

[lvjonok]

Let me add something from my experience of working with MJCF models. I, personally, didn't find many reasons behind holding very similar scene.xml from each Mujoco model and, thus, developed a tool, which automatically generated it given z position of floor intended.

I think a similar solution will be even better in long-term for general-purpose usage

[kevinzakka]

In Menagerie, the scene.xml is mostly there to have consistent plane, lighting, free-camera settings and for overall aesthetics. I think it's more proper to load the canonical model and have a separate scene variant. In general, the variant option would be very useful :)

[stephane-caron]

Thank you both for your feedback! Then it seems the best way forward is to allow it but not make it the default.