Problem witg accessing 3D Slice files.

When reading a 3D SLCF by

```
slc = sim.slices.get_by_id("SMOKE_DENSITY_3D")
smoke_density_array = slc.to_global()
```

I get "ValueError: could not broadcast input array from shape (20,20,21) into shape (20,20,20)".

Looks like it has to do with the cell_centered = True state.
If I manually override it by
`slc.cell_centered = False`
it works.





