Variable Permeability and coupling free-flow-porous-media problems

The coupling manager as of now requires the spatial parameters of the porous-media problem to provide a permeabilityAtPos() function. However, when modeling evaporation-driven mineral precipitation, the permeability is not simply depending on the location, but on the solution of the porous-media problem. However, one does not really want to give the porous-media problem solution to the coupling manager.

I guess evaporation-driven mineral precipitation is one of the fancier applications for our free-flow-porous-media coupling and might become more popular, so we should think about a solution to this problem.