Rename fare_zone_id column

[r-akemii]

In this PR we rename the variables fare_zone_id and fare_zones in stops and fare_zones transit tables, explaining its relation to stops.txt file in GTFS. We also remove an unnecessary column.

Modifications include:

• renames transit_zone to zone_id in stops.sql and fixes documentation
• renames fare_zone_id to transit_fare_zone in fare_zones.sql and stops.sql and fixes documentation
• removes references to foreign key fare_zone_id in fare_rules.sql
• fixes variable name in queries in transit/lib_gtfs.py and transit/transit_elements/stops.py
• fixes tests in tests/aequilibrae/transit
• fixes Coquimbo example to assign these modifications

[r-akemii]

Hey @pedrocamargo, I want to check one thing with you before opening this PR. A couple of days ago I asked you if I couldn't remove the fare_zone table. You told me it was necessary because it associates the fare zones from stops with the TAZs, but we also have this same information in the stops table, i.e. we have the same information in two different tables. My question is: can we make it simple and keep this info in only one table?

[pedrocamargo]

This is wrong.

The fare zone should be transit_fare_zone (and appear in the fare AND stop table), while **zone_id ** should appear only in the stop table and contain the ZONE_ID for the zoning layer