This method is used by our visualizers to render n-dimensional grids which cannot be embedded into R^n by duplicating vertices which would have to be mapped to multiple points at once (think of grids on rectangular domains with identified edges).
subentities(0, grid.dim)relation for the flattened grid.
The coordinates of the codim-
Maps the indices of the codim-
grid.dimentities of the flattened grid to the indices of the corresponding entities in the original grid.