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

Added init-finalize also for vectors.
Require gather/scatter operations instead of direct vector/matrix access.
Rewritten the interpolation.
Added ParallelIndexSet and DOFMapping to be used also in other backends.

allow order() as memberfunction of gridfunctions and redirect free function to this memberfunction if available