Add out-of-system walking transfers to navigation engine

[idreyn]

From Ethan on slack:

For instance, right now to go from West Newton to Longwood, it recommends going from the Worcester Line to the Orange Line then the Green Line, when you could make the same trip by walking from Lansdowne to Kenmore with one transfer.

There are a good handful of stations within walking distance of each other (Copley/Back Bay being the classic example) for which a walking transfer would save a lot of time, especially in a world where there's no fare penalty for leaving the system to do so. To add this to the Explorer, we'd probably want to gather (from Google Maps or similar) a set of walking distances between reasonably close stations (<=0.25mi?) and seed neighbor stations into the initial search states.

We should be mindful of wheelchair++ users when launching this feature — taking into account hostile street design, elevator access, etc. when including/excluding station pairs and finalizing walk times.

[idreyn]

The data has been collected in https://github.com/transitmatters/station-walking-distance and we just need to import it into the explorer now!