img
an antsImage, either continuous or mask-like. This will
define the space where to sample voxels from. If this image is
continous (i.e., a T1), voxels will be sampled from all nonzero
voxels.
perLabel
logical, if true the input image must be multi-label
and the the output will have nsamples from each label (i.e., 200
samples from each label)