(Re-)Implement CFL criterion
As a first step of porting the decoupled/sequential models to the new structure, it would be a good thing to implement a CFL criterion for the time step control for the current porousmediumflow models. Add good start is e.g. a CFL is e.g. the 1p_tracer test (https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/tree/master/test/porousmediumflow/tracer/1ptracer) that uses an explicit Euler for the transport but currently has a constant time step that is small enough for the test. CFL would be a big improvement.
@martins Maybe you are the best to deal with this?