Merge branch 'feature/vetorization-unstructured-grids-cherry-pick' into 'master'

ref:extensions/dune-codegen

-   Make it possible to do vectorization on unstructured grids
-   Move most things concerning permutations into the interface classes of the
    sumfact kernel
-   Add test cases for all variations of facedir and facemod

See merge request [extensions/dune-codegen!285]

  [extensions/dune-codegen!285]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/285

The realize_output interface method was not really necessary and just adds a
layer of indiretion.

Due to a bug in dune-geomtry the volume calculation is not accurate enough. For
now we just remove all volume calls from the weak formulation.

Merge branch 'feature/geometry-mixins' of ssh://gitlab.dune-project.org:22022/extensions/dune-codegen into feature/geometry-mixins