Smoothing factor. Can be scalar, in which case the same sigma
is applied to each dimension, or a vector of length dim(inimg) to
specify a unique smoothness for each dimension.
sigmaInPhysicalCoordinates
If true, the smoothing factor is in
millimeters; if false, it is in pixels.
FWHM
If true, sigma is interpreted as the full-width-half-max (FWHM)
of the filter, not the sigma of a Gaussian kernel.