tree_to_graph_pm() with POD

Here's a version of the code with the new translation method along with some preliminary documentation. Hopefully, it's more suitable now. 

I still can't think how to test, without actually writing graph_pm_to_tree() to prove a round-trip is possible (and I remain unconvinced that writing that sub is even possible). I suppose it'll be preliminarily enough to use LectroTest to generate random trees, and analyze them within Graph.pm to ensure the results remain both directed and acyclic.