Bug: ResizeSampler uses scipy.ndimage.zoom which can result in incorrect resamples

[atbenmurray]

We should move to a better resampler, such as skimage.resize, that samples from / at pixel / voxel centres. Details to follow, but this issue affects downsampling.

[tvercaut]

Note that this is related to #334 The implementation of the resampling should be transparent to the user and should ideally be efficiently running on the GPU when feasible.