NF - started working on new framework for local tracking

    Added basic outline of tracking framework in dipy.tracking.localtrack
        including DirectionGetter, TissueClassifier some code to hold it all
        together.
    Added example direction getter for PeaksAndMetrics in dipy.reconst._peakdg
        so PeaksAndMetrics can inherit DirectionGetter attributes.
NF - added PythonDirectionGetter
NF - added LocalTracking class

    This class pulls together everything needed for tracking including:
        direction getter, tissue classifier, seeds, affine, and more
NF - Probabilistic Direction Getter - in python for now
Started on an example to demonstrate new api
BF - add except -1 to direction getter method so python errors don't get
     ignored by cython
BF - small issues in direction getters
TST - A few tests for direction getter / tissue classifier
BF - fix voxel size computation in LocalTracking
BF - python3 stuff
fixup! BF - fix voxel size computation in LocalTracking
RF - using smaller sphere for small speedup in api example
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.
BF: yield a copy of F if stepsB==0.
RF: removed unused variable.