Minimal subscriber cookbook recipes

This package contains a few different strategies for creating nodes which receive messages:

• lambda.cpp uses a C++11 lambda function
• member_function.cpp uses a C++ member function callback
• not_composable.cpp uses a global function callback without a Node subclass

Note that not_composable.cpp instantiates a rclcpp::Node without subclassing it. This was the typical usage model in ROS 1, but this style of coding is not compatible with composing multiple nodes into a single process. Thus, it is no longer the recommended style for ROS 2.

All of these nodes do the same thing: they create a node called minimal_listener and subscribe to a topic named topic which is of datatype std_msgs/String. When a message arrives on that topic, the node prints it to the screen. We provide multiple examples of different coding styles which achieve this behavior in order to demonstrate that there are many ways to do this in ROS 2.