Py-trees

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* ``failure_is_running decorator`` (meta).

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* [backend] capture paralell policy details

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* [docs] using ros2 launch, not ros2 run for the tutorials

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* [docs] usage instructions

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* [backend] feed the viewer blackboard access data, `27 <https://github.com/splintered-reality/py_trees_ros_viewer/pull/27>`_
* [all] machinery for activity streams, `28 <https://github.com/splintered-reality/py_trees_ros_viewer/pull/28>`_

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* [infra] update to make use of usability / performance improvements in ``py_trees_js v0.5.1``