Merge branch 'feature/nonlinear-matrix-free' into 'master'

Feature/nonlinear matrix free

Depends on PDELab merge request https://gitlab.dune-project.org/pdelab/dune-pdelab/merge_requests/183.

* [x]  stationary CG
* [x] stationary DG

Important notes:

- The nonlinear_matrix_free_dg_numdiff throws a time out. I already reduced the grid size to 4x4 and I don't want to go any smaler. Maybe we just delete this test? I think it's ok if we only test numdiff in CG since we want to use symdiff anyway. Opinions?
- Instationary matrix free needs some changes in PDELab. Should be postponed until it is supported there.


Closes #26 .

See merge request !29