Mixed grid support for InterfaceValues

[AbdAlazezAhmed]

The current implementation uses two FaceValues and which face is here is determined by the topology of the grid (vertex_vertex_neighbor or face_face_neighbor). IIUC there's no guarantee that for an interface between two types of cells one type will always be here and the other will be there. So, with the current implementation we need n² number of InterfaceValues for a grid with n types of cells (that if we don't cache what types share an interface beforehand).

I assume we can just change which cell is here in InterfaceCache as InterfaceIterator contains the grid.