Add depthfirst to TraversalOrder to help with plotting

[diegozea]

That will help when creating Arrays for plotting node attributes, such as adding names/labels to internal nodes.

Cheers

[richardreeve]

@diegozea - as far as I remember we originally had depth first and breadth first, but then it was pointed out to me that there were three kinds of depth first ordering, preorder (which is what I always used to call depth first), inorder and postorder. There's a description of them here, for instance. Is preorder what you meant? If so, is that okay?

What we definitely need is documentation for that enum though!

[diegozea]

Hi @richardreeve ! I have tried the three, pre, in, and post order, and they don't match the node depth first order used for plotting. It looks like plotting and map_depthfirst are using some of the reverse orderings; is that possible?

[mkborregaard]

I intended plotting to use preorder but might have implemented it wrong

[richardreeve]

Or I might have!