Improve definition of Fare Products

[isabelle-dr]

Problem

There is confusion around what a fare product is in GTFS. Two sources of confusion were brought-up to our attention:

First, the definition of fare_products.txt is:

To describe the different types of tickets or fares that can be purchased by riders.

When in reality, some fare products represent virtual products that can't be purchased (by themselves) by riders, such as a transfer cost.

Second: although fare_poduct_id's type is ID and not Unique ID, it's unclear in the spec that several Fare Products can be grouped under the same fare_product_id, and under which conditions this should be done to ensure an efficient fare design.

Solution

The file definition should be modified to include virtual products that can't be directly bought by riders.
The file definition should give insights on when fare products should be grouped under the same ID.
The definition of fare_poduct_id should be modified to include something like:

Multiple records in fare_products.txt may have the same fare_product_id, in which case all fare products with that ID will be applied when referenced from another file.