Kluge around problems with picking nodes in HyperViewer on Windows.
I will have to fix the bug in PyOpenGL argument passing to get picking working right. Meanwhile, I put in a simple work-around hack: when you pick a graph edge in the spanning tree displayed by HyperViewer, the node it leads to is picked instead. (The edge has a negative integer ID, while the node it leads to is positive.) This also gives the ability to "zip" along edges of the graph, which I kind of like. It's not a very good substitute for picking nodes, but might be worth keeping even after node picking works. We can also define our own semantics for picking edges in the graph...