ENH: Change the classifier in BRAINSABC

Add a KNN based tissue classification component to BRAINSABC.

After each EM estimation phase, a set of clean plugs (i.e.
areas where there is greater than 66% chance being that structure)

Are chosen and a Fuzzy KNN is used to re-estimate probabilities of each region.

ComputeEMPosteriors and ComputeKNNPosteriors are set in one function
called ComputePosteriors, that means KNN is run at each iteration of EM
loop.
Also, the dirtyThresholdedLabels image is the one that is used in
ComputeKNNPosteriors, since it has all labels (not only foreground regions).