Computes the global image threshold using Otsu's method by minimizing the
inter-class variance.
Inputs:

histogramCounts : is the histogram data of the image, that can be calculated
using `imhist` function.

total: is the total number of pixels that can be counted using the
`numel` function.

Output:
ll : is the level of the global threshold in [0 1]

Example:

consider the grayscale image is Ig

>> otsu(imhist(Ig),numel(Ig))
>> 04414