Otsu threshold implementation

[anlava]

We now have a problem with this part of code:

delta_mean += mean - m;
let variance = delta_mean * delta_mean
                / (count - w).value_as::<T>().unwrap()
                / w.value_as::<T>().unwrap();

if variance < last_variance  {
                break;
            }

It seems that in some moment 'delta_mean' is too small which causes problems in further comparison of 'variance' and 'last_variance'.