Kinematic mode fails in gazebo using Joint interfaces: SetPosition and SetVelocity

To test, uncomment [this block](https://github.com/PositronicsLab/Pacer/blob/master/example/interfaces/gazebo.cpp#L247) and set [kinematic flag to true](https://github.com/PositronicsLab/Pacer/blob/master/example/models/links/vars.xml#L7).
