Use simple smoothing in find_cut_slices rather than a gaussian filter

Fixes #233.
But then don't you need to test all the edge cases (pun not intended), i.e. depending on how many neighbors the 1 has?

What if I made the 'expected' array a bit less magical, for example by defining 

```python
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.