BF - fix voxel size computation in LocalTracking
NF - added trilinear interpolation function for tracking
RF - clean up tracking code and move it to tracking.local
fix - don't need localtrack.pxd anymore
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.