def add_roi(roi)
      raise ArgumentError, "Invalid argument 'roi'. Expected ROI, got #{roi.class}." unless roi.is_a?(ROI)
      @structures << roi unless @structures.include?(roi)
    end
def add_roi(roi)
      raise ArgumentError, "Invalid argument 'roi'. Expected ROI, got #{roi.class}." unless roi.is_a?(ROI)
      @structures << roi unless @structures.include?(roi)
    end
Adds a ROI instance to this StructureSet.

 @param [ROI] roi a roi instance to be associated with this structure set