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Stripped down and rebuilt for ROS2:
* [behaviours] the familiar subscriber and action client behaviours
* [blackboard] the exchange, mostly unmodified
* [infra] colcon build environment
* [trees] simpler communications, just one serialised tree snapshot, watchers do the rest
* [watchers] revamped 'blackboard' and new 'tree' watcher
What's missing:
* [logging] the basic mechanisms have moved to py_trees, the rosbag implementation is to come