Interpolation support

[julianrojas87]

Is valid GTFS to define stop_times with both empty arrival_time and departure_time. However we cannot have Connections with empty departure or arrival times.

According to the spec, this implies that the consumer needs to interpolate these stop times, which is difficult in this case given that Connections are created in a streaming way.

A possible fix could be to do this as part of the pre-processing step that already takes place to order stop_times. Reusing an existing tool that handles this scenario, might make things easier.