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

in as dictionary on the python side

in dune-python)

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

parameter from paramBase

perhaps should be part of the Model class?)

Also make vortex euler problem work