def structure_numbers
      numbers = Array.new
      @structures.each do |s|
        numbers << s.number
      end
      return numbers
    end
def structure_numbers
      numbers = Array.new
      @structures.each do |s|
        numbers << s.number
      end
      return numbers
    end
Gives the numbers of the structure set's associated structures.

 @return [Array<Integer>] the numbers of the associated structures