Add 1D floodplain schematization to kinematic wave routing

[verseve]

For local inertial river routing a 1D floodplain schematization has been implemented, and it makes sense to add this functionality also to the kinematic wave routing. The current kinematic wave solution assumes a fixed alpha value. This will not work properly with a 1D floodplain schematization, so some research work (alternative solver) is required.