RF - Reworked integration to be more clear. Using explicit terminate callable
     in generate_streamlines now.
TST - remove old generate_streamline test
RF - took out reference and track_to_directions in integration
BRK RF - updating tracking tools to use new api
RF - re-factored interpolate to be more clear and rename as markov
TST - skipping tracking/interfaces.py in test for now
RF - minor cleanup of odds and ends
RF - update trilinear_interp to take float32 data and have better behaviour at
     image edges.
RF - Better checking TriLinearInterpolator. index can be strided, and
     voxel_size must be contagious.
RF - fixup tracking to work without directions attribute
TST - fix up tracking tests and add test for OdfWeightedTracker
RF - make steppers callable to avoid having to write stepper.take_step all the
     time