Py-trees

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* [visitors] winds of change visitor and logging demo

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* [console] fallbacks for unicode chars when (UTF-8) encoding cannot support them

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* [trees] don't use multiprocess on setup if not using timeouts

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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

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* [infra] shortcuts to types in __init__.py removed (PR117_)
* [bugfix] python3 rosdeps
* [idioms] pick_up_where_you_left_off added

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* [idioms] oneshot added
* [bugfix] properly set/reset parents when replacing/removing children in composites