Use neod3.

Lots of things still broken at present, especially:

- styling not connected to the main app, but operates on its own
- arrow routing is very unstable, and produces a flickering effect
  with densely connected graphs
- arrow drawing sometimes produces invalid SVG including infinite
  coordinates, probably due to a divide-by-zero error.

We also need to delete a large amount of code used by the original
visualisation structure.