Impact Mode: Refine the visualisation

Besides just studying the impact on shortcuts within one neighbourhood boundary, the user can see how traffic changes across roads in the whole map. This works by finding the best route before and after changes for every origin/destination "OD" pairs, then counting routes per road.

Features:

- [x] Demand model mvp: random OD pairs
- [x] Demand model v2: Use same NPW data for Scotland. Need to detect country and decide how to clip that data. For those special areas, prebake maps with everything bundled in? Or prebake clipped OD datasets that match map names?
- [x] Just do the calculation naively
- [x] Display relative results
- [ ] Experiment with showing absolute vs relative counts
- [ ] Let user change the main road penalty
- [ ] Change default main road penalty to > 1.0 ?
- [x] Click one road to see paths through it that changed
- [ ] Consider CSV export of the data (being careful about communicating the absolute vs relative change)

Not important, just a stray idea: Consider some kind of differential "spider map"; see https://github.com/Urban-Analytics-Technology-Platform/od2net/issues/35