Propose changes to FPTF

[andaryjo]

The current FPTF version does not satisfy all the requirements needed to be able to fully adapt it for this client.

Let's propose some enhancements! Especially:

• Add line and direction to stopover.
• Add line to leg.

You can see all the changes I had to make here: https://github.com/andaryjo/trias-client/blob/c605157d535231a550c021b85df70eb216f668f8/src/types/fptf.ts

[derhuerst]

cc @juliuste

[andaryjo]

Contributed proposals regarding line and direction to the FPTF V2 discussion issue here: public-transport/friendly-public-transport-format#61 (comment)

[andaryjo]

Contributed proposals regarding decoupling of line and mode and introduction of an unknown mode here: public-transport/friendly-public-transport-format#62 (comment)