variant. applyDeformation/convertDeformationType/Space were assuming voxel
alignment between deformation field and reference image.

passed. Used by fsl_convert_x5

complete?

nonlinear.DeformationField

- CoefficientField can now represent a generic b-spline coefficient
field. Cleaned up fnirt.py, other tweaks and comments

nonlinear module. Expand x5 module to handle non-linear. Rename x5 functions
to be more generic.