Trip explorer supports transfers

[rudiejd]

Is your feature request related to a problem? Please describe.

My most common trip involves a green to red line transfer. I think at least one transfer is common for many Greater Boston residents, and it would be awesome to see this represented on the trip explorer!

Describe the solution you'd like

Display data for a trip with transfers, for example Kendall => Boylston

Additional context

Figuring out all the shortest trips will likely require something from the Djikstra's algorithm family, unless someone has an easier idea!! We can precompute and store the shortest paths for each trip, but we should probably use a database for storing them (that would push the limits of a JSON file, lol).

How would we handle the shortest paths being disconnected? It'd be annoying to recompute shortest paths every day based on closures, but that could be possible.

[rudiejd]

Some other thoughts here:

• the real travel time for an inter-line trip includes time spent waiting for the transfer train to arrive, so it'd be awesome to include schedule data here as well
• Subway to Bus or to CR transfers would also be awesome, but that could be a later addition