Use simple smoothing in find_cut_slices rather than a gaussian filter

Fixes #233.
> What if I made the 'expected' array a bit less magical, for example by defining

> value_with_3_neighbors = (1 + 3 * neighbor_weight) / (1 + nb_neighbors * neighbor_weight)

> and replacing 0.72727273 by value_with_3_neighbors. I would do something
> similar with the value for 4, 5 and 6 neighbors.

Than I would be happy :).

The problem with precomputed values is that when things break a few years
down the line, it's harder to diagnose the break down, because you don't
know how the reference has been generated.