Document states of the robot

[blegat]

Document what each states of q represent in BipedRobot.mechanism.
cc @ThiagoAlvesLima

[ThiagoAlvesLima]

The robot has 21 states, namely eleven configurations (q) and ten velocities (v). The eleven configurations are:

• The first four are a quaternion representing the orientation of the hips in relation to the world/base frame.

• The next three represent the position of the hips (in x,y,z, coordinates) in relation to the world/base frame.

• Then, the next two represent the angles of the right and left tight tights of the robot, respectively, around the local y-axis of the joints between the tights and the hips.

• Finally, the next two represent the angles of the right and left legs of the robot, respectively, around the local y-axis of the joint between the legs and the tights.

Then, the ten velocities are the derivative of the configurations. Since the 3-d orientation of the hips is represented in quaternion coordinates, there is one less velocity than position.