Newton Relaxation
@benjamin.terschanski: Take a look at https://gitlab.dune-project.org/dune-fem/dune-fem/-/blob/master/dune/fem/solver/newtoninverseoperator.hh?ref_type=heads#L116 how to add a parameter, and then https://gitlab.dune-project.org/dune-fem/dune-fem/-/blob/master/dune/fem/solver/newtoninverseoperator.hh?ref_type=heads#L372 how to use it.