Accept Structured grids in VTK files

[jhdark]

Should just require some minor changes here:

        if isinstance(in_data, pyvista.UnstructuredGrid):
            grid = in_data
        elif isinstance(in_data, pyvista.core.composite.MultiBlock):
            # To handle multiblock like pvd
            if hasattr(pyvista, "_VTK_SNAKE_CASE_STATE"):
                pyvista._VTK_SNAKE_CASE_STATE = "allow"
            else:
                # Compatibility with 0.47
                pyvista.core.vtk_snake_case._state = "allow"
            number_of_blocks = in_data.number_of_blocks
            assert number_of_blocks == 1
            b0 = in_data.get_block(0)
            assert isinstance(b0, pyvista.UnstructuredGrid)