It would be nice if we can be notified when a node in the flowgraph has finished by adding a method `GraphListener.onNodeFinished(FlowNode node)`

For AtomNode, we only get notified in the GraphListener before the node starts execution but never when it ends.

For steps that have a block, we have the dedicated BlockEndNode but at the time it is called, the status of the node is not yet set.

While we can use the getParents() of a FlowNode and process the parent nodes, it means we are not able to react in a timely manner to the end of a node. E.g. parallel branches would only be processed once the complete parallel step is finished and the FlowEndNode can never be processed this way.