read.nrrd.header cannot cope with 4D CMTK deformation fields

[jefferis]

The key issue is that the space dimensions is assumed to consist of 3 vectors at various locations in the code, but in these 4D fields contains a none entry.

NRRD0004
# Created by NrrdDisplacementFieldWriter at Thu Nov 15 08:06:35 EST 2018
type: float
dimension: 4
encoding: raw
space: right-anterior-superior
sizes: 3 269 324 311
space directions: none (0.75,0,0) (0,0.75,0) (0,0,0.75)
kinds: vector domain domain domain
labels: "Vx;Vy;Vz" "x" "y" "z"
endian: big
space origin: (0,0,0)