Update Mujoco Simulation: Remove u_mujoco, allow for setting gains in real time

[Zolkin1]

Right now the Mujoco simulation requires the user to create a u_mujoco vector that is used to populate the mujoco inputs. Now that the PDFeedforward control takes in joint names we should be able to match the inputs to the message. We may need to make an assumption on the ordering of the mujoco inputs (like P, D, FF).

Some robots, like the Unitree ones, allow for on the fly adjusting of the gains, we should update the mujoco simulation to support this type feature.