ParallelNode threshold dynamic based on number of children?

[zflat]

I would like to be able to use the ParallelNode and specify that all nodes must succeed. However, every time I add or remove a child node in the XML, the success_threshold would also need to be updated. I would like a way to make the threshold dynamic based on the number of children.

Here is an implementation to address this: #363

[zflat]

@facontidavide Thanks for accepting this change! I appreciate the documentation you added and cleanup you did in ce6503a and ef68cf1.