reimplement interpolate function with averaging

Replace Dune::Functions::interpolate with own implementation based on the dune-functions implementation. Here, I want to allow non-smooth (non-continuouse) functions in the interpolation. Thus simple nodal-interpolation does not work. A first strategy is implementated, namely node-averaging. A counter is added that counts how often a values is added to a DOF and after the interpolation the DOF value is divided by this counter value.

Other strategies that could be added:

• Clement type interpolation (using local L2 projection)

• Evaluation in super-convergent points, i.e. average over evaluations in the element barycenters

• Least-squares approximation

• Therefore, the interface of interpolate() must be extended to support some kind of strategy flag.