Parameter group and documentation
In https://dumux.org/docs/doxygen/master/runtime-parameters.html add a description about the parameter model when dealing with multiple instances of the same class and a global parameter tree. We use group prefixes then to set different parameters for the different instances. This could be more transparent for most classes but definitely needs to be documented in the doc page.
We should also think about flexibility and better design to pass param groups. It would be good if the parameter group could be changed after construction. Or maybe we could think about some factory pattern that allows to set the parameter group of a class instance and use this pattern everywhere. Maybe parameterized classes could provide constructor overloads that take the param group as the first parameter (using some named type).