Add Gaussian smoothing before undersampling
Add Gaussian smoothing before undersampling

Gaussian smoothing is performed previous to the scaling operation if
zoom factor < 1. This reduces the aliasing introduced by the
undersampling operation.

Choice of sigma according to:
    Cardoso et al., "Scale factor point spread function matching:
    beyond aliasing in image resampling", MICCAI 2015

NifTK/NiftyNet,NifTK/NiftyNet,NifTK/NiftyNet,NifTK/NiftyNet