Minimal publisher examples

This package contains a few different strategies for creating short nodes which blast out messages. The talker_timer_lambda and talker_timer_member_function recipes create subclasses of rclcpp::Node and set up an rclcpp::timer to periodically call functions which publish messages. The talker_without_subclass recipe instead instantiates a rclcpp::Node object without subclassing it, which works, but is not compatible with composition, and thus is no longer the recommended style for ROS 2 coding.