Fix issues in python bindings due to changes in dune-vtk

This MR fixes an issues introduced by changes in the extension module dune-vtk.

A dependency to dune-vtk was introduced in !467 (merged) using the internal gridfunction type-erasure Dune::Vtk::Function. This utility was recently rewritten and renamed into Dune::Vtk::GridFunction and moved into a new header dune/vtk/gridfunctions/gridfunction.hh.