Consider implementing Phansalkar local thresholding

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From @LeeKamentsky on September 25, 2015 12:31

See https://github.com/stelfrich/imagej-ops/blob/7ef98f4101700213d49ac11184e4278f26b7a2bd/src/main/java/net/imagej/ops/threshold/localPhansalkar/LocalPhansalkar.java for an implementation in ImageJ and Phansalskar N. et al. Adaptive local thresholding for detection of nuclei in diversity stained cytology images. International Conference on Communications and Signal Processing (ICCSP), 2011, // 218 - 220 for the citation.

A local thresholding method based on a calculation done on the local mean and variance:

# Phansalkar recommends k = 0.25, r = 0.5, p = 2 and q = 10
img_mean = convolve(img, strel_disk(radius))
img_stdev = sqrt(convolve((img - img-mean)**2, strel_disk(radius)))
t = img_mean * (1 + p * exp(-q*img_mean) + k * ((stdev/r) - 1)

Copied from original issue: CellProfiler/CellProfiler#1587