def narrays(sort_slices=true)
      n = Array.new
      @volumes.each do |volume|
        n << volume.narray(sort_slices)
      end
      return n
    end
def narrays(sort_slices=true)
      n = Array.new
      @volumes.each do |volume|
        n << volume.narray(sort_slices)
      end
      return n
    end
Creates an array of 3D NArrays from the binary volumes of this instance.
 Note that the master volume (if present) is excluded, and if no volumes
 associated, an empty array is returned.

 @return [Array<NArray>] an array of 3D NArray from the associated binary volumes