[vtk] Allow shared_ptr arguments for addSolutionToVTKWriter

This commit introduces an additional signature for addSolutionToVTKWriter that takes grid function space and data vector as shared pointers. This is necessary because of the delayed nature of VTK export: Data sets are registered in a first step and written in a second step. Currently PDELab implicitly requires both steps to happen in the same scope, as otherwise the registered function objects can be destroyed before written into a file. The additional signature allows to pass shared pointer objects that the VTK functions take ownership of. Internally, this required the BlockVector of the ISTL vector backend to store the grid function space as a shared pointer.