Humanoid Upper Torso MJCF/URDF Adjustments

[thomasonzhou]

The humanoid coordinate frames and link for the robot should be adjusted to conform to https://www.ros.org/reps/rep-0120.html

For reference, some exisiting MJCF files:

• https://github.com/google-deepmind/mujoco_menagerie
• https://humanoids.wiki/w/Robot_Descriptions_List

[thomasonzhou]

Subtasks:

• Change zero position to be folded for single arm (not upright)
• Investigate + document differences between common humanoid and OpenArm
• Evaluate other coordinate parameterizations:
  • Denavit-Hartenberg (current)
  • Screw Theory