Put property definitions in tests (examples/applications) in separate header
To make another step away from the problem-does-everything-mentality that was common (and a necessary evil) in dumux 2,
I suggest to put the property definitions into a separate header. There are not connected in any way to the
Problem class, so IMO they shouldn't be defined in the problem header. For the purpose of re-usability they also shouldn't be directly coded in the main file.