Adds strategy to update stop.vehicle_types

[hbruch]

Summary: The optional extension field stop.vehicle_type indicates which route_types are served at a stop. It may be used e.g. to visualize stops in maps accordingly.

This strategy collects all route_types via all stop_times/trips at each individual stop. In case multiple route_types are served, it tries to map extended route_types to GTFS standard route_types. If still then route_types are inconsistent, a missing value of -999 is set.

• [x] Run the unit tests with mvn test to make sure you didn't break anything
• [ ] Format the title like "Fix #<issue_number> - short description of fix and changes" (unchecked, as this is not a fix)
• [ ] Linked all relevant issues (unchecked, as searching for vehicle_type did not list any relevant issues)
• [x] Added unit test for new functionality