Merge pull request #11 from pal-robotics/max-curr

Add ActuatorCurrentLimit message.
Deprecate service limiting single actuator current

The topic interface should be favored, and there's already a reference controller
implementation in pal_ros_controllers/current_limit_controller.
Deprecate original current limiting message

The message in pal_control_msgs/ActuatorCurrentLimit is favored because:

- It has correct semantics, we are limiting _actuator_ currents, not _joint_
  currents.

- It allows to limit multiple actuators.