Add fallback decorator for 3D images

We don't support images greater than 2D, so fall back on ndimage
Actually, pretending I am a new reader here, this is really unclear. Could you add a few comments above this line, e.g.

```
# our functions don't work in 3D, so for 3D input we should fall back on `scipy.ndimage`.
# Additionally, we want to use a cross-shaped structuring element of the appropriate dimension
# for each of these functions. The `default_callback` provides all these.
```