Filter departures and arrivals by line id

[RBeerDevelopment]

Currently the /stops/:id/departures and the /stops/:id/arrivals only take the direction and the duration as filter arguments.
However, sometimes it would also be interesting (or easier) to filter by a specific line (eg. ?line=u5) or do this in addition to the existing filters.

[derhuerst]

This is tricky, because the VBB HAFAS API used by vbb-rest…

• …doesn't necessarily support a "line ID" that matches what people would intuitively call a "line" (e.g. S8, M8 or X1). – Instead, a "line" seems to be a group of trips grouped under a name in the schedule data that HAFAS has been fed with; So wether the "lines" make sense seems to depend on the structure of VBB internal schedule data.
• …currently doesn't return those HAFAS "line IDs", even though e.g. the Deutsche Bahn HAFAS API does.

Also, even if the VBB HAFAS API returned proper line IDs, hafas-client wouldn't parse & expose them (just as id with fallback to slugg(name), which is not usable for filtering):

https://github.com/public-transport/hafas-client/blob/f530a30fe019f3c8e19a4eb0f15fa99776db6f7e/parse/line.js#L8-L20

But hafas-client already supports filtering by such "line IDs":

https://github.com/public-transport/hafas-client/blob/f530a30fe019f3c8e19a4eb0f15fa99776db6f7e/index.js#L60

Let's wait until the VBB HAFAS API returns them, then we can implement this for vbb-rest.

[derhuerst]

In any case, having this filtering by "line ID" in hafas-rest-api would be beneficial for all HAFAS APIs that return them – specifically, db-rest would gain this functionality. If you want to implement this, please submit a Pull Request! 🙌