Add support for using walk routing from OpenTripPlanner
Currently our OpenTripPlanner plugin assumes the graphs in the server only include a transit graph (from GTFS feeds) and no OpenStreetMap data, thus producing "as-the-crow-flies" walking paths, and complements by calling out to GraphHopper to find walking routes and does OTP routing only from/to the nearest transit stop to the start and end. We did this as the intention was to provide a default GNOME-operated OTP instance and make it scale better by not including street data. But now that we use third party services, and some (most?) will include full data for their region, it makes more sense to take advantage of that (as it also can give better results also). So we should make "full" routing using coordinates and parse the walking instructions from OTP the default mode and have as an extra option the current way of doing external walk routing (the Djerba OTP instance e.g. does not contain OSM data). This should be done after #203 (closed)