coefficient fields

can get away with it. Can't be bothered figuring out how to adjust srcToRefMat
in convertDisplacementSpace, so am just not supporting it

encodes. Might be moved to a different location

between fsl.transform.nonlinear and fsl.data.image.

used by the DisplacementField class.

logic, instead of re-implementing similar logic, and premat handling is all
in one place.

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

coefficient field to displacement field. Check for TOPUP intent codes

are fine. Generating a slightly less incorrect (but still incorrect) v2w
affine for the former. Bug in test script

convertDisplacementSpace