add unified access for the implementation of DiscreteFunction

Improved test and range initialization of DiscreteFunction

extend the DiscreteFunctionTest to test for construction with std containers

Add range type to solution() and oldSolution() functions

make AMDiS GridFunction differentiable w.r.t. dune-functions definitions. Make DiscreteLocalFunction copyable

Removed the direct solver specialization ofr double types in favour of the separate initialization in the creators