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.
Good catch here, thanks for that. We should return `F[:stepF].copy()` because I think `F` get's reused here and `F` and `F[:stepF]` share memory.