Example 7: Remove empty callbacks in controllers and update docs

[christophfroehlich]

Background

Related to ros-controls/ros2_controllers#1474

IMHO we could remove the empty callbacks

ros2_control_demos/example_7/controller/include/ros2_control_demo_example_7/r6bot_controller.hpp

Lines 65 to 72 in 40c59ee

	controller_interface::CallbackReturn on_cleanup(
	const rclcpp_lifecycle::State & previous_state) override;
	controller_interface::CallbackReturn on_error(
	const rclcpp_lifecycle::State & previous_state) override;
	controller_interface::CallbackReturn on_shutdown(
	const rclcpp_lifecycle::State & previous_state) override;

or at least add a comment, that they are just for demonstration purposes and are optional.

At least we should update the docs, that the overrides of the lifecycle transitions are optional and not mandatory

ros2_control_demos/example_7/doc/userdoc.rst

Lines 336 to 365 in 40c59ee

	The controller plugin for the tutorial robot is a class called ``RobotController`` that inherits from ``controller_interface::ControllerInterface``. The ``RobotController`` must implement nine public methods. The last six are `managed node <https://design.ros2.org/articles/node_lifecycle.html>`__ transitions callbacks.
	1. ``command_interface_configuration``
	2. ``state_interface_configuration``
	3. ``update``
	4. ``on_configure``
	5. ``on_activate``
	6. ``on_deactivate``
	7. ``on_cleanup``
	8. ``on_error``
	9. ``on_shutdown``
	.. code-block:: c++
	class RobotController : public controller_interface::ControllerInterface {
	public:
	controller_interface::InterfaceConfiguration command_interface_configuration() const override;
	controller_interface::InterfaceConfiguration state_interface_configuration() const override;
	controller_interface::return_type update(const rclcpp::Time &time, const rclcpp::Duration &period) override;
	controller_interface::CallbackReturn on_init() override;
	controller_interface::CallbackReturn on_configure(const rclcpp_lifecycle::State &previous_state) override;
	controller_interface::CallbackReturn on_activate(const rclcpp_lifecycle::State &previous_state) override;
	controller_interface::CallbackReturn on_deactivate(const rclcpp_lifecycle::State &previous_state) override;
	controller_interface::CallbackReturn on_cleanup(const rclcpp_lifecycle::State &previous_state) override;
	controller_interface::CallbackReturn on_error(const rclcpp_lifecycle::State &previous_state) override;
	controller_interface::CallbackReturn on_shutdown(const rclcpp_lifecycle::State &previous_state) override;
	// private members
	// ...
	}

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[saikishor]

I agree 👍🏽