Mixed grid support for `InterfaceValues`

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.