Composite elements not fully supported in the Unity plug-in

MuJoCo supports the [`composite`](https://mujoco.readthedocs.io/en/latest/XMLreference.html#body-composite) XML element that instructs the compiler to add multiple bodies, joints, tendons and/or constraints to create soft materials. This “macro” functionality means that as opposed to most other MJCF elements, composites cannot be associated with a single specific runtime model element. Therefore, binding the composite element itself is ill-defined. However, you can still import composite elements through the load-save-load workflow, and they are imported as the atomic bodies / joints / tendons / constraints.