Added MeasuringPoint.natural_key() method
Added MeasuringPoint.natural_key() method

CodeforLeipzig/luftqualitaet_sachsen,CodeforLeipzig/luftqualitaet_sachsen,CodeforLeipzig/luftqualitaet_sachsen,CodeforLeipzig/luftqualitaet_sachsen,plumps/luftqualitaet_sachsen,plumps/luftqualitaet_sachsen