Add 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.

• 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)
• Added unit test for new functionality

[CLAassistant]

All committers have signed the CLA.

[leonardehrenfried]

Hello, I'm the new maintainer of this library and will do a bit of renovation work.

If you're still interested in getting this merged, can you rebase it onto the latest master? You will probably need to change a few imports in the test since I migrated to Junit 5.