[GTFS-Fares v2] Add fare_leg_join_rules.txt (first iteration)

[tzujenchanmbd]

This PR is based on Google's fare_leg_join_rules.txt proposal. At the workgroup meeting on February 27th, the following consensus was reached:

• Predicates for from_network_id and to_network_id will be added first, followed by the addition of other predicates such as from_stop_id and to_stop_id.
• from_network_id and to_network_id are required fields, and initially will be restricted to the same network_id. We will consider relaxing this for different network_id later.

Changes in this PR

• Add Effective Fare Leg in term definition
• Add fare_leg_join_rules.txt
• Modify network_id, from_area_id, to_area_id, from_timeframe_group_id, to_timeframe_group_id in fare_leg_rules.txt for effective fare leg

For previous discussions please see the working group meeting note and issue#346

Please go through the changes and feel free to share your thoughts/questions here.