Add option to compute metroscore for accessible transit networks only

[agupta01]

🚀 The feature, motivation and pitch

It's a point of frequent discussion at transit agencies about how and when to go about making their public transit networks accessible. Most recently the NYC MTA outlined its accessibility goals in its 20-year plan.

The goal of this feature is to add an accessible flag to the metroscore computation, something like:

def metroscore(
    origin: Point, 
    departure_time: datetime, 
    duration: float, 
    wheelchair_accessible: bool = False
):
    ...

So users who want to analyze metroscores for those who require wheelchair access can do so by flipping the flag to True:

ms = metroscore(origin_point, "Tuesday 4PM", 60, wheelchair_accessible=True)

This will subset on the part of the transit network that is wheelchair accessible before computing the metroscore.

Alternatives

Alternative would be to manually recreate the transit network for accessibility and calculate metroscore using it (but this is a lot of work).

Additional context

GTFS data has a wheelchair_boarding attribute in the stops.txt file.