NF: added trilinear interpolation to ThresholdTissueClassifier.
BF: When stepsB was 0, it returned B[-1:0:-1]: all points of the list B except idx 0. Now, none is returned.
BF: yield a copy of F if stepsB==0.
RF: removed unused variable.
Merge pull request #6 from gabknight/add_trilinear_interp

NF: added trilinear interpolation to ThresholdTissueClassifier