improved documentation a bit
switched registration to degrees and mm
Best motion parameters yet, but still have some work to do
ShearImageKern Bugfix, OTSU threshold, smaller rigid reg test size

- ShearImageKern was incorrectly bounds checking
- OTSU was in biascorrect, but is now part of ndarray_utils
- Rigid registration doesn't need to be tested on such large images 32x
  is sufficient