NTFS: fare_zone management

[pbougue]

So far, fare_zone are only managed in 2 places in NTFS:

• In stops ( https://github.com/CanalTP/navitia/blob/dev/documentation/ntfs/ntfs_fr.md#stopstxt-requis ).
• In fares application conditions ( https://github.com/CanalTP/navitia/blob/dev/documentation/ntfs/ntfs_fare_extension_fr.md#farescsv-optionnel ).

No object is managed, we only deal with a fare_zone_id. So we use it as the name when displaying is necessary.
This can lead to some problems, if we have multiple contributors with fare_zone that are named the same.

To be decided when case appears:

• Do we create an new file describing fare zones or do we simply add a fare_zone_name in stops/fares?
• Can we imagine multiple fare zones at the same stop_point? (a stop point used by 2 transport agencies that would have fare zones)