def add_structure(structure)
      raise ArgumentError, "Invalid argument 'structure'. Expected ROI/POI, got #{structure.class}." unless structure.respond_to?(:to_roi) or structure.respond_to?(:to_poi)
      @structures << structure unless @structures.include?(structure)
    end
def add_structure(structure)
      raise ArgumentError, "Invalid argument 'structure'. Expected ROI/POI, got #{structure.class}." unless structure.respond_to?(:to_roi) or structure.respond_to?(:to_poi)
      @structures << structure unless @structures.include?(structure)
    end
Adds a Structure instance to this StructureSet.

 @param [ROI, POI] structure a Structure to be associated with this StructureSet