def narray
      if @images.length > 0
        narr = NArray.int(@images.length, @images.first.columns, @images.first.rows)
        @images.each_index do |i|
          narr[i, true, true] = @images[i].narray
        end
        return narr
      end
    end
def narray
      if @images.length > 0
        narr = NArray.int(@images.length, @images.first.columns, @images.first.rows)
        @images.each_index do |i|
          narr[i, true, true] = @images[i].narray
        end
        return narr
      end
    end
Builds a 3D dose pixel NArray from the dose images belonging to this
 DoseVolume. The array has shape [frames, columns, rows] and contains
 pixel values. To convert to dose values, the array must be multiplied
 with the scaling attribute.

 @return [NArray<Float>] a 3D numerical array