What this MR does / why does DuMux need it:
Add a simple hyperelastic model and test
does the new code follow the style guide?
do the test pipelines pass? (see guide on how to run pipelines for a merge request)
is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in