Feature list for tests in dumux

The old and outdated feature list (https://dumux.org/doxygen/2.12/a06492.html) is going to be replace by a new one which will be put into the new examples folder in dumux. Here we can collect ideas on what features we would like to have on the list. For some we may already have examples in the test folders, for others we need to create new examples or extend examples to include them.