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