Boundary boxes
Boundary boxes

- Now computes boundary box for gpx_track, then calculates the
  intersection with map boundaries
- Edge cases are still unhandled

TODO:
- Check to see if a point in the track boundary is an edge case
- Add a buffer to the track boundary for visibility
- Use GDAL bindings to auto merge selected maps
- Trim merged maps to track boundary
- Add option to download the right topo files

mbsperry/find_topo.rb