Extend export_to_vtk for tensor product prism/wedge elements

Currently it only possible to visualize tensor product prisms with `equi_dist_nodes = true` and equal polynomial degrees in `Nplot`. In order to extend this functionality to unequal polynomial degrees, it is probably necessary to generalize `SUD_to_vtk_order` and functions therein, though I don't know if vtk even support anisotropic prisms. The first step toward adding support for `equi_dist_nodes = false` would be to solve #215 by adding a method for subdivion into smaller elements. In #214 we removed a specialization of `MeshData_to_vtk` for prisms. It may be possible to modify this function to serve one or both of these issues.