Bring effort interfaces back to work in Gazebo

[marcoesposito1988]

As can be seen in commit 3376577, removing spring_effort makes the Gazebo simulation work again with effort controllers.

How can we do this in a clean way? Shall we put a yaml parameter to lwr_hw_sim?

[carlosjoserg]

I will update this next week as well. The idea is to have the same behavior in both lwr_hw_sim and lwr_hw_real, so the plugin will expose both PositionJointInterface and EffortJointInterface, emulating strategy 10 and 30.

For EffortJointInterface is clear, is what exists today that emulates strategy 30.

For PositionJointInterface there are two possible variants based on position commands:

1. Use a PID within the plugin to compute efforts that are sent to the simulation.
2. Use the SetPosition().

I prefer 2, even when it is not advised for simulation.

[carlosjoserg]

I believe this is partially solved by #42