single matching_edges function

[yakra]

With separate matching_simple_edges, matching_collapsed_edges and matching_traveled_edges, we iterate thru the entire set of vertices & check for matches 3 times.
Combine into one function, and iterate only once, match multiple edge types at the same time.

[yakra]

Better yet, get rid of edge sets and a separate function to find them altogether, instead just writing them to the file as they're found. Nope. Need to know how many matching edges we have in order to write the header.

[yakra]

Trying out #217 first will make this less of a PitA to rewrite. In C++ at least. Python's not as big a deal.