Calibration Test for Track A

- Model: Assume velocity may be difference across different pieces of
  track
- Dijkstra now returns total *cost* of the shortest path. We use this to
  setup known distances between many adjacent pairs of sensors then use
  these known distances for calibration. See dijkstra's unittest to see
  which pairs we probe.
- Calibration involves running train 47 on the inner loop a total of 10
  times. It's pretty easy to run. Just set the train somewhere on the
  inner loop and run the program.