Sobel magnitude filter is not normalised

[StRigaud]

Sobel kernel filter return the gradient magnitude using the Sobel filter for each (z,y,x) dimensions.

The output is computed as:

M = sqrt(dx^2 + dy^2 + dz^2)

However the Sobel filter is not normalised. Although this does not change the dynamic nor the result, the return values will be different than most Image Analysis library (e.g. scikit-image).