Gridfunction compatible to dune::functions::gridFunctions
Make the gridfunction compatible to the gridfunction concept defined in dune-functions. Therefore, we need to introduce entitySets for abstract gridfunctions, e.g. AnalyticGridFunction and ConstantGridFunctions. Some questions need to be answered, e.g.
- What is the common EntitySet of a FunctorGridFunction
- Is it necessary to introduce DerivativeTraits or can be stick with the defaults