Use simple smoothing in find_cut_slices rather than a gaussian filter

Fixes #233.
It would be more efficient (and less readable :) ) if you did something like

<pre>
smoothed_arr =  arr / (scale * neighbor_weight)
smoothed_arr += arr[1:]
...
smoothed_arr *= neighbor_weight
</pre>

Indeed, as each '+=' line, you are doing a costly multiplication.