How can I access the angle of a specific joint in the walker model?

Hi, thank you for this wonderful repo!

I'm trying to use the walker model for some RL projects. And I want to access the angles of some specific joints.

I think the observation is a dictionary with a 14-dim orientation array, a 1-dim height array, and a 9-dim velocity array. I also looked at the **walker.xml** file. But I still don't know the meaning of each element of these arrays. Can anyone give me some advice on that (e.g. which file should I refer to, what is the order of the joints)?

Another issue also discussed the similar topic: https://github.com/google-deepmind/dm_control/issues/453. Seems that we need to calculate the joint angles using orientations of the body parts. But the problems is that in that issue, the observation seems to be 21-dim instead of 14-dim, different from what I obtained using the code below.

```Python
r0 = np.random.RandomState(42)
env = suite.load('walker', 'walk', task_kwargs={'random': r0})
state = env.step(action)
obs = state.observation
```


Any help/discussion is appreciated!