NF: added trilinear interpolation to ThresholdTissueClassifier

The code is very efficient! The design is simple and good. I want to implement some things to get into it. 

It is more or less the first time I code using cython. Your feedback is very welcome. I wanted to add a new TissueClassifier, but first I noticed your #TODO for the trilinear interpolation.
We don't need ijk anymore.