Add support for car ferries

[VillePihlava]

Goal / high level use-case

Ferries can have cars aboard. The goal is to add support for using ferries with cars.

Describe the solution you'd like

The gtfs field bikes_allowed works in a similar way to how this feature could work. The proposed solution would be to adapt the solution for cars. This would involve adding a cars_allowed field to gtfs. In the future, a similar solution could be considered for trains that can have cars aboard.

Additional context

As of writing this issue, many details of the implementation are still somewhat unclear and will be appended once the work moves forward.

[vesameskanen]

We have seen some problems when trying to route long bike journeys with ferry connections. If such itineraries are computed using transit routing, access, egress and max transfer duration must be very long. That in turn may slow down routing so radically that it becomes unusable.

Maybe such routes should be searched using street routing methods.

[ravolka]

HSL / Ville has started to work on this

[optionsome]

We got a confirmation that both schedule and non-schedule based ferries need to be supported. We don't have a need for cars on trains in the short term.

[miklcct]

The issue of access modes is currently being discussed which gives a more general alternative on these fields, as they no longer fit all the use cases. google/transit#466