Add stops.stop_access field

[tzujenchanmbd]

This PR adds a stop_access field in stops.txt to indicate how the stop is accessed for a particular station. Please refer to this proposal for details.

We expect this field to clarify the following:

• Whether routing riders to stops requires passing through designated entrances.
• That the station-stop hierarchy may include stops not within the same physical structure

Adding this field is also the first phase of our three-phase plan to improve station modeling. For details on previous discussions, please refer to issue #438.

[tzujenchanmbd]

What about adding the following clarification for stop_access = empty, per this suggestion:

When its parent station does not contain any entrances/exits (location_type=2), data consumers should consider this stop as not part of the main station structure or enclosed area.

[jfabi]

What about adding the following clarification for stop_access = empty, per this suggestion:

When its parent station does not contain any entrances/exits (location_type=2), data consumers should consider this stop as not part of the main station structure or enclosed area.

Do we have a good sense about how frequently producers include entrances for their parent stations? Even for us at the MBTA, we include them (and pathways) in full for our metro stations, but for the most part have not added them for our many regional rail parent stations. I'm not sure that the exclusion of station entrances, at this time, can imply one behavior or the other—after all, this ambiguity about the definition of a parent station is part of the rational for stop_access.

Of course, consumers will (and already do) have to make assumptions in those cases—from a survey in our market, it looks like OTP and the major commercial apps are mixed in whether they route involving entrance-less stations to the parent or the child.