nonlinear.DeformationField

is no longer assumed to be aligned with the reference in voxels - it is
only assumed to be aligned to the reference in world coordinates.

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.