routes.agency_id: add check

[AntoineAugusti]

This condition is not enforced at the moment.

Agency for the specified route. This field is required when the dataset provides data for routes from more than one agency in agency.txt, otherwise it is optional.

From routes.txt

[antoine-de]

This could be related to rust-transit/gtfs-structure#126 since the typed ids would enforce all ids to be valid.

But there is still a bit of work on the PR and I'm not sure when i'll be able to work on it

[fchabouis]

We could just add a check that if there are more than one agency, agency_id must be present. If not, we can outpout an Error.

[AntoineAugusti]

@fchabouis What I had in mind