Merge branch 'feature/dg-operator' into 'master'

ref:dune-fem/dune-fem-dg This MR needs
[dune-fem!287]

See merge request [!4]

  [dune-fem!287]: dune-fem!287
  [!4]: gitlab.dune-project.org/dune-fem/dune-fem-dg/merge_requests/4

interface in dune-fem.

Merge branch 'feature/dg-operator' of ssh://gitlab.dune-project.org:22022/dune-fem/dune-fem-dg into feature/dg-operator

limiterPolynomialOrder.

choosing different discrete function spaces.

dune-fem-dg.

Also got standard Burgers example to work

Merge branch 'feature/dg-operator' of ssh://gitlab.dune-project.org:22022/dune-fem/dune-fem-dg into feature/dg-operator