Improve intermodal route only handling

[jfbischoff]

fixes #2802
In the SwissRailRaptorConfig, there is now a switch to define the desired behavior for handling routes, where feeder-Station-feeder legs return the lowest cost pt route:

• forbid: These routes are discouraged, instead the router will try to find a route with at least one pt leg. If nothing is found, null is returned (= which later translates into the fallback mode, usually walk)
• avoid: These routes are discouraged, instead the router will try to find a route with at least one pt leg. If nothing is found, the router will return a feeder-station-feeder leg
• allow: These routes are allowed if they produce the lowest cost. This could be useful in combination with on-demand modes
• returnNull (deprecated): This mimics the behavior that was currently produced. This behavior is basically "allow", followed by an overwriting of this route by null. If feeder modes are defined at very low cost, this can lead to very unwanted behavior.

forbid is currently the default. This should have the lowest impact on existing scenarios, yet still remove the buggy behavior that could have occured before.