This field is only ever set when using motion generators, i.e. the position- or
velocity interface of the FCI. When sending a target value, the FCI sends you
back the generated trajectory trajectory in `q_d`, `dq_d` and `ddq_d`.

However franka_gazebo does not yet support any motion generator interface. Via
torque control interface the robot sets these values to zero indicating that no
motion is generated, which we replicate in franka_gazebo simulation.

Note that `q_d` is a special case. This updates also in torque control mode and
also during guiding to reset motion generators on replanning attempts. For
brewity we just set it to `q` in franka_gazebo