TODO: Fix data collection quadruped

[giulioturrisi]

Given the fact that mujoco needs torque for its sys id routine, we should save the commanded torque on the quadruped example. This is done in the manipulator example but not in the one of the quadruped.

To be future proof and reproducible, the gain used for collection as well should be saved in the dataset.

TODO:

• compute commanded torque and save it (NOT ANYMORE, see below)
• save gains

[giulioturrisi]

We decided to direclty use PD from mujoco for sys-ID, for stability reason. So we need only to save Kp and Kd we used to obtain the dataset