What this MR does / why does DuMux need it: This MR introduces FluxTraits that for now only check if the AdvectionType is of the type stationary velocity field. If so the velocity calculation is adapted. This leads to correct velocity (output) for the tracer model e.g. It also changes the reference solution, but only for the velocity, as the new solution now shows the correct values at the inflow/outflow.
Which issue this MR fixes: fixes #782 (closed)
Special notes for your reviewer: Should be finished from my side. @timok I hope this is what you had in mind. Make sure to test with debug/assertions.