USD Integration

[abhihjoshi]

Three files:

• mujoco/python/mujoco/usd_exporter.py
• mujoco/python/mujoco/usd_component.py
• mujoco/python/mujoco/usd_utils.py

The USD exporter class handles all the overall USD file and materials. The USD component files handles all the objects in the scene.

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[yuvaltassa]

Let's delete the usd_ prefix to avoid stuttering. usd/component.py etc.

[yuvaltassa]

This needs to be rebased. Probably better to just close this PR and start a new one at head? The discussion is long and mostly irrelevant...