Matlab's median function slow for big 3D stacks

[MPIBR-tushevg]

one can use a O(n) selection algorithm from:
https://de.mathworks.com/matlabcentral/fileexchange/29453-nth-element
to calculate median of a 3D image matrix in the following way

[h, w, z] = size(img);
tmp = reshape(img, h * w, z);
tmp = tmp';
tmp = fast_median(tmp);
med = reshape(tmp, h, w);