Adds support to multiple transit agencies

[r-akemii]

In this PR we add support to import more than one transit agency per GTFS file. We also modified how stops are numbered and its agency_id.

Modifications include:

• allows the import of multiple transit agencies per feed
• removes stops numbering based on transit agency
• updates column selection for GTFS files
• updates examples in docs (plot_import_gtfs.py and plot_public_transit_assignment.py): removed agency argument
• updates link to GTFS documentation in modeling_with_aequilibrae/public_transport.rst and on database specifications (aequilibrae/project/database_specification/transit/tables)
• updates tests in paths, project, and transit

[r-akemii]

An initial thought was import transit agency data first, and have a workflow similar to the one that exists in __load_date (transit/gtfs_loader.py). I had an initial thought of loading all data for one agency at a time (like load all data for agency A, then load all data for agency B), but I don't think it's a good idea.