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.