Previously, it was available only for elements.
The following question appears: what does the codim-parameter in
template unsigned int subIndex (const Codim::Entity& e, int i, unsigned int codim) const
mean? Is it the codimension wrt to the grid, or wrt to the entity e? I did not find this documented anywhere, and therefore made my own choice. The 'codim' in this patch is the codimension with respect to the grid.