Support GTFS #70
Description
Is your feature request related to a problem? Please describe.
At the moment, Vahrplan solely depends on DB-Hafas. This is a problem, since the Hafas instance may be unreliable (see https://www.github.com/public-transport/hafas-client/issues/331).
Describe the solution you'd like
Support parsing of GTFS(-RT) data and use open data/feeds provided by Delfi (GTFS Static and GTFS-RT). It would be cool if the user could choose their favorite data source
Describe alternatives you've considered
Use of other hafas data sources with hafas client other than the db one. However, none of them are as thorough as db hafas.
Additional context
None