def composition_string
      composition.sort.map do |k, v|
        v == 1 ? k.to_s : "#{k}#{v}"
      end.join('.')
    end
def composition_string
      composition.sort.map do |k, v|
        v == 1 ? k.to_s : "#{k}#{v}"
      end.join('.')
    end
A string representation of a unit based on the atoms it's derived from
 @return [String]
 @api public