def add_contour(contour)
      raise ArgumentError, "Invalid argument 'contour'. Expected Contour, got #{contour.class}." unless contour.is_a?(Contour)
      @contours << contour unless @contours.include?(contour)
    end
def add_contour(contour)
      raise ArgumentError, "Invalid argument 'contour'. Expected Contour, got #{contour.class}." unless contour.is_a?(Contour)
      @contours << contour unless @contours.include?(contour)
    end
Adds a Contour instance to this Slice.

 @param [Contour] contour a contour instance to be associated with this slice