BF: accelerate registration and vector field inversion algorithms (this subtle detail in ANTS was responsible for the 1% Jaccard index difference between Dipy's module and ANTS), now we are able to closely replicate even ANT's energy profile. I validated this change by running 306 registrations and verified that the results are closer to ANTS both in anatomical region overlap and CSF/WM/GM overlap. Updated regression tests accordingly.
Merge branch 'syn' into expected_cc