How can I get concrete definitions for the results from `physics.named.data`

Take the "quadruped" domain as an example. It is known that we can get rough understandings from the `quadruped.py` that `self.named.data.sensordata` contains sensor readings as follows:
```
self.named.data.sensordata
Out[33]:
FieldIndexer(sensordata):
0 imu_accel [ 7.2 ]
1 imu_accel [-2.65 ]
2 imu_accel [ 22.7 ]
3 imu_gyro [ 0.448 ]
4 imu_gyro [ 0.411 ]
5 imu_gyro [-0.751 ]
6 velocimeter [-0.206 ]
7 velocimeter [-0.0947 ]
8 velocimeter [-0.249 ]
9 force_toe_front_left [-62.4 ]
10 force_toe_front_left [ 30.4 ]
11 force_toe_front_left [-1.41 ]
12 force_toe_front_right [ 85.1 ]
13 force_toe_front_right [ 20.3 ]
14 force_toe_front_right [-101 ]
15 force_toe_back_right [ 1.19e+03]
16 force_toe_back_right [-291 ]
17 force_toe_back_right [-725 ]
18 force_toe_back_left [ 256 ]
19 force_toe_back_left [-23.1 ]
20 force_toe_back_left [-936 ]
21 torque_toe_front_left [ 0.232 ]
22 torque_toe_front_left [ 0.757 ]
23 torque_toe_front_left [-0.187 ]
24 torque_toe_front_right [-0.256 ]
25 torque_toe_front_right [-1.01 ]
26 torque_toe_front_right [ 0.139 ]
27 torque_toe_back_right [ 2.94 ]
28 torque_toe_back_right [-19.2 ]
29 torque_toe_back_right [ 13 ]
30 torque_toe_back_left [-28.8 ]
31 torque_toe_back_left [ 36.2 ]
32 torque_toe_back_left [-9.05 ]
33 center_of_mass [-0.00071 ]
34 center_of_mass [-0.0362 ]
35 center_of_mass [ 0.82 ]
```
Although these names provide basic information, I still don't know the concrete meanings for each item. For example, it is hard to infer which `imu_accel ` is for x-axis.

quadruped.py と、issue に示されている self.named.data.sensordata の出力から始めます。センサー定義をたどり、imu_accel、imu_gyro、force、torque、center_of_mass の各エントリを含む、グループ化された各値の軸と物理的な意味を特定します。報告されたインデックスについて具体的な意味を文書化し、ユーザーが各結果を解釈できるようになれば完了です。

索引モデルが issue の本文から書いたものです。