Investigate articulation root API

[ChengshuLi]

• What is the “correct” place to attach articulation root API? Isaac seems to change behavior: from object to base link
• From this link, the correct place seems to be the object prim.
• Do we still need to accommodate the bug of omniverse (dc getting inconsistent joint order)? -> some objects articulation root API is at the object level, and some are at the base link level? Maybe we can set it always to be at the object prim level
• For non-jointed single body objects, if we don’t attach articulation root API, then phyx -> usd update is too “lazy”. Can we always attach an articulation root API even in this case? Any performance regression?