Adds closed-loop support + PAL Robotics' KANGAROO

[louislelay]

This PR adds PAL Robotics’ KANGAROO robot. The model relies on closed-loop linkages, so I also added support for those.

Screencast.from.15-10-25.14.44.56.webm

It’s not ready to merge yet: the arms aren’t configured correctly, and the motion shows significant jitter/glitching. When retargeting to a robot with closed-loop linkages and running the visualization script, I hit this error:

(.venv-gmr)  14:39:37   (louis/closed-loop-support-and-kang-implementation)
~/retargeting/GMR$ python scripts/vis_robot_motion.py --robot pal_kangaroo --robot_motion_path ../kang_data/data.pkl --record_video --video_path ../kang_data/kang_fight.mp4
Recording video to ../kang_data/kang_fight.mp4
Traceback (most recent call last):
  File "/home/louislelay/retargeting/GMR/scripts/vis_robot_motion.py", line 33, in <module>
    env.step(motion_root_pos[frame_idx], 
  File "/home/louislelay/retargeting/GMR/general_motion_retargeting/robot_motion_viewer.py", line 118, in step
    self.data.qpos[7:] = dof_pos
ValueError: could not broadcast input array from shape (30,) into shape (26,)

I’m opening this PR so you, @YanjieZe, can take a look at the code/results and point me in the right direction: why the jitter/glitches, and how you’d prefer to resolve the visualization issue. It seems caused by passive joints being recorded even though they aren’t actuated. Should we record only actuated joints, or do you have another approach in mind? Thanks in advance!