Add support for calendar_dates usage
Some datasets (e.g. switzerland) contain mostly all service calendar information in the calendar_dates.txt and not the calendar.txt file. We have to find a way, to better support this method in the isochrone datasets.
By now we use the onebusaway gtfs tool to copy calendar_dates to the calendar table, but this seems not be the best solution (bus_nodes outdegree gets very high for some reason... not sure where the problem exactly is, but we have to look at this issue more closely)