Use simple smoothing in find_cut_slices rather than a gaussian filter

Fixes #233.
For completeness, this is what I have done, I think.

Also about the naming of the functions, I am fine adding 3d explicitely to `_smooth_array` or `_fast_smoth_array` if you think it is better. Maybe something like `_smooth_array_three_first_dims`? Better suggestions welcome! 

In this case should the 'public' function smooth_img be renamed too?