support missing calendar.txt and missing calendar_dates.txt

[Jannis R]

The calendar_dates.txt table can explicitly activate or disable service by date. It can be used in two ways.

• Recommended: Use calendar_dates.txt in conjunction with calendar.txt to define exceptions to the default service patterns defined in calendar.txt. If service is generally regular, with a few changes on explicit dates (for instance, to accommodate special event services, or a school schedule), this is a good approach. In this case calendar_dates.service_id is an ID referencing calendar.service_id.
• Alternate: Omit calendar.txt, and specify each date of service in calendar_dates.txt. This allows for considerable service variation and accommodates service without normal weekly schedules. In this case service_id is an ID.

[Jannis R]

Published as gtfs-utils@5.