With a Dune::VTKSequenceWriter I would like to have code like

    Vector x;
    VTKSequenceWriter<...> writer(...);
    VTKBasisGridFunction<...> function(..., x, ...);
    writer.addVertexData(function);

    for (int i = 0; i < level; ++i) {
        ... // refine grid, solve PDE, change size of x
        writer.write(static_cast<double>(i));
    }

As the size of "x" is only correct in the for-loop, it cannot be checked
when constructing the "writer". However the "function" needs to be setup
outside the loop as well. So the VTKBasisGridFunction constructor should
not check the size of "x".