Add a simple proximal newton solver

changed the description

Didn't @patrick.jaap_at_tu-dresden.de just write such a solver? Do we need yet another one?

Yes, indeed! In dune-solvers: dune-solvers!63 (merged)

The MR is ready to be merged from our perspective. It is already in use for our current projects. It should be highly modular and adaptable to all kind of problems. You only need to create a second order solver object, and a smooth energy object. Everything else should work with the given dummy methods.

Yes thanks, I'll close this MR then!

closed