includeWheelchairData to tripAccessibilityFilter

I like your idea, Nick, and after doing the grunt work of following all
the bean parameters around to add my original boolean, it was fairly
straightforward to change those to an enum.

Now, instead of an 'includeWheelchairData' request parameter, there's a
'tripAccessibilityFilter' request parameter. Valid values are:

DO_NOT_INCLUDE (the default): retrieve all relevant trips, but don't
include the trip.wheelchairAccessible field

ALL: retrieve all relevant trips, with accessibility information
included in the unofficial field trip.wheelchairAccessible

ACCESSIBLE_OR_UNKNOWN: retrieve all relevant trips with
trip.wheelchairAccessible=ACCESSIBLE or
trip.wheelchairAccessible=UNKNOWN, but not
trip.wheelchairAccessible=NOT_ACCESSIBLE

ACCESSIBLE_ONLY: retrieve only relevant trips with
trip.wheelchairAccessible=ACCESSIBLE

This applies to these API calls: arrival-and-departure-for-stop,
arrivals-and-departures-for-stop, block, block-instance,
estimate-current-vehicle, plan-trip, trip, trip-details,
trip-for-vehicle, trips-for-agency, trips-for-location, trips-for-route,
vehicle and vehicles-for-agency.