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.