I don't really see the point in that as the surface density is a region specific constant which does not depend on the queried cell at all. But yesterday evening I've also come to the conclusion that for the other methods it's better to directly pass them the region index instead of trying to hide it. I'm thinking on something like

```c++
oilProps.mu(muDestBuf, numCells, pvtRegionIdxForCell, cellPressures);
```

that way, the API would stay agnostic of the grid cells. (but obviously it still needs to become aware of the PVT region...)