Accept only TransitStops for TransferEdges

These four commits demonstrate a new paradigm I just made up called type safety driven design.
Improve type safety of GtfsStopContext

These four commits demonstrate a new paradigm I just made up called type safety driven design.
Fix compilation errors

These four commits demonstrate a new paradigm I just made up called type safety driven design.
Disallow chaining of TransferEdges

These four commits demonstrate a new paradigm I just made up called type safety driven design.
Add stop ids to more legs to make Beter Benutten MMRI people happy
Allow paths starting and/or ending with transfers