add utilities following the Datastore API semantics

[derhuerst]

The Datastore API of transit.land looks like a promising approach to expressing transit data in a way that's relevant for many practical types of computation & analysis on transit data.

[derhuerst]

In this representation, each possible move between two stops has a unique edge, called a ScheduleStopPair. Each edge includes an origin stop, a destination stop, a route, an operator, and arrival and departure times. Each edge also includes a service calendar, describing which days a trip is possible. Accessibility information for wheelchair and bicycle riders is included, if available. Some of this data is normally split across multiple GTFS tables, but is here denormalized for simpler access: each edge contains enough information to get from one stop to another, to another, and finally to your destination.