Suggestion for new unit: kg/m2

There is a suggestion for a new unit 'kg/m2' by corinna.rebmann@kit.edu.

Check it out at https://sms-cv.helmholtz.cloud/sms/cv/api/v1/units/136/.

Please have a look @corinna.rebmann1, @heiko.thoss, @karl-heinz.haefele, @nils.brinckmann, @paul.remmler, @r.kunkel, @romy.foesig, @stephan.schreiber