Fix some issues with the VirtualizedGF in the python bindings

The dune-grid python bindings include the type-erasure grid-functions from dune-vtk. As far as I understand, it is there to allow conversion of grid functions on the type-erasure type of dune-vtk such that regular grid functions can be used in the writer. Since dune-vtk has changed its type-erasure implementation, the hack in dune-grid is not working anymore and leads to compile errors in completely other modules.

This MR tries to fix the construction of the type-erasure classes from dune-vtk. In the process, I noticed that the wrapper type PyGridFunction actually has no meaning anywhere else, and might be removed. But since I am not sure whether someone has used it, I have just deprecated the header and the class in this MR.

Discussion

It would be better to move the whole dune-vtk part out of dune-grid into dune-vtk. What needs to be added there to make it work?