New folder "properties"
For the sake if isolating the property system and making it easier to find stuff in Dumux, I believe it might be helpful to create a new top level folder properties
that contains
properties
|- properties.hh
|- propertysystem.hh
|- base.hh
|- grid.hh
|- discretization
|- box.hh
|- cctpfa.hh
...
i.e. all files that are concerned with setting basic properties. The question is what to do with the model.hh
headers but I suggest that they remain part of the respective model folder, and only the "base"-property headers are separately structured.