Add support to more than one transit agency in GTFS feeds

[r-akemii]

Currently, AequilibraE only supports a single transit agency for each feed import.

If our agency.txt file has more than one agency, all of them are saved under the umbrella of the agency defined in the import
transit = data.new_gtfs_builder(agency="agency_name", file_path="gtfs_file_path").

It would be nice if AequilibraE supported more than one transit agency at a time, without the requirement of providing a transit agency name during the feed import.