Use simple smoothing in find_cut_slices rather than a gaussian filter

Fixes #233.
> I assumed the leading underscore in the function more or less implied that it
> wasn't supposed to be used across modules.

No, it only implies that end-users shouldn't be using it.

> Also, smooth_img always removes non-finite values (via _smooth_array).

Hum, that's a good point. Maybe we want to be using _smooth_array than.

> Maybe if we really don't want to create a temporary NiftiImage we can use
> _smooth_array with fwhm='fast'?

Agreed.