def contour_image
      img = NArray.byte(columns, rows)
      contour_indices.each_with_index do |contour, i|
        img[contour.indices] = i + 1
      end
      return img
    end
def contour_image
      img = NArray.byte(columns, rows)
      contour_indices.each_with_index do |contour, i|
        img[contour.indices] = i + 1
      end
      return img
    end
Applies the contour indices of this instance to an empty image
 (two-dimensional NArray) to create a 'contour image'.
 Each separate contour is indicated by individual integers (e.g. 1,2,3 etc).

 @return [NArray] a contoured numerical array