frequencies.txt: support exact_times=0

[derhuerst]

As of #2, gtfs-via-postgres supports all frequencies.txt rows with exact_times=1, but not those with exact_times=0.

Frequencies.txt represents trips that operate on regular headways (time between trips). This file can be used to represent two different types of service.

• Frequency-based service (exact_times=0) in which service does not follow a fixed schedule throughout the day. Instead, operators attempt to strictly maintain predetermined headways for trips.
• A compressed representation of schedule-based service (exact_times=1) that has the exact same headway for trips over specified time period(s). In schedule-based service operators try to strictly adhere to a schedule.

It is unclear how to model frequency-based services in the existing arrivals_departures & connections views, which are currently designed to handle schedule-based services only.

The minimal-effort option would be to expose exact_times in arrivals_departures & connections, but consumers would have to take care to either a) filter exact_times=0 rows out, or b) make sure they handle both cases correctly.