PT routing

Walking + PT has been started in this repo. Just starting a list of improvements.

# UI

- [ ] Just make a nice UI to explore GTFS data. See routes on a map, see timetables per stop, etc. Look for inspiration. Old ideas in https://github.com/dabreegster/bus_spotting and https://github.com/a-b-street/abstreet/issues/372.

# A-to-B routing

- [ ] Configurable start time
- [ ] Display text route directions nicer. "Walk 5 minutes", "wait 4 minutes", plumb info about the route taken
- [x] Correctness -- only ever 1 stop per trip? Impossible
- [ ] Use route shapes for display (and see how much space they need)
- [ ] Get more precise about linking walking and transit steps

# Isochrone

- [x] Implement it

# Data model

- [ ] Decide requirements for days of the week, service exceptions, etc
- [ ] Think through serveless-friendly ways of getting GTFS data

# Performance

- [ ] Can an A*-like heuristic help for PT routing? Edit: yes, but it loses optimality
- [ ] Get base file size down, without GTFS. London is 370MB, but the PFB is 90MB
- [ ] Smaller GTFS data. London is around 300MB right now
- [ ] More compact NaiveTimes?

# Refactoring

- [ ] Figure out what frontend and backend components are generally shareable
- [ ] Specifically use in https://github.com/Hussein-Mahfouz/meet-me-halfway/