Improve transit JSON structure

[kuanb]

An improved transit JSON structure would support key additional attributes needed including:

• dwell times by stop
• speeds broken out by segments

No need to reinvent the wheel here. Conveyal has a nice structure for describing new lines. I've already sketched out conversion logic from Conveyal's GTFS editor's custom lines and stops export JSON to edges and nodes dataframes loadable by peartree.

See more on those notes, here: http://kuanbutts.com/2019/07/14/conveyal-transit-json/