RF: Fixes to `NEWIMAGE::volume` data access in `vecreg`
Replaced calls to NEWIMAGE::volume[t](x, y, z)
with NEWIMAGE::volume(x, y, z, t)
- the former pattern causes unnecessary copies of entire 3D volumes (specificallly spline coefficients) to be made. As this was happening for every voxel, this was having a substantial impact on performance.
Edited by Paul McCarthy