Aggregate frequency without splitting by agencies

[takohei]

Currently, frequency (in aggregated_routes) is always split into separate records per agency.
For GTFS feeds that contain multiple agencies, I would like to be able to aggregate frequency across agencies (i.e., without separating by agency).

This likely requires changes in both the GTFS-GO GUI and the gtfs_parser processing.

This figure shows an example of frequency being split by agencies (Maebashi City, Japan).