Fix FlowNodeWrapper#getNodeType so it can return NodeType.STEP

[stuartrowe]

Fixes #352.

Changed the order in which the NodeType criteria are checked. Evaluating node instanceof AtomNode before checking PipelinelineNodeUtil.isStage(node) avoids the situation where an AtomNode is considered a NodeType.STAGE just because it has an associated LabelAction.

Testing done

It's not clear to me what impact this fix actually has, but it was clearly wrong before.

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