Add support for realtime occupancy-information

[lassetyr]

OTP should have support for occupancy-information as provided in GTFS-RT and SIRI.

A first step could be returning the information as an attribute in the response, but it could potentially also be included as an optional cost-parameter in a trip-request. Trip-requests could then be able to prioritize trips with available seating.

[mpaine-act]

Our ridership would love this innovation. Also, if there was a way to publish (daily?) historical trip/stop occupancy, that could feed into this as well. Both could work as weights related to each other.

[barbeau]

@mpaine-act FYI, there is is a draft GTFS extension spec for historical trip/stop occupancy proposed at google/transit#240. Comments there are welcome!