Follow up from #648. To make another step away from the problem-does-everything-mentality that was common (and a necessary evil) in dumux 2, we plan on putting the property definitions into a separate header. Previous work accomplished in #648: * Separate properties into `properties.hh` for the examples (!1933) * Separate properties into `properties.hh` for the dumux-course#24 Current goals: * [x] Separate properties into `properties.hh` for the dumux-lecture dumux-lecture#18 * [ ] Separate properties into `properties.hh` for the tests This doesn't have to be in one go. If someone edits a test anyway feel free to do this change as well.