def structure_names
      names = Array.new
      @structures.each do |s|
        names << s.name
      end
      return names
    end
def structure_names
      names = Array.new
      @structures.each do |s|
        names << s.name
      end
      return names
    end
Gives the names of the structure set's associated structures.

 @return [Array<String>] the names of the associated structures