add enumeration value "kg/kg" to "unit" of the elements "quantity", "minQuantity", "maxQuantity", "defaultQuantity"

[hurni]

cp. blw-ofag-ufag/eCH-0261#56

To indicate usage of plant protection products, certain business partners use percentage and others indicate the absolute mass of two products as befitting their needs. Although percentage can be calculated, relevant information is lost that can be used in other use cases (e.g. if an absolut maximum of a product is allowed on an acreage, etc.)
medium/long term the business partners may be persuaded to harmonize. But in the meanwhile, digiflux needs to be able to use "kg/kg" as an additional unitType.

@RyanStuder-Work: kannst du bitte kurz die Erklärung und den Lösungsvorschlag gegenprüfen? Danke

[AFoletti]

This is a minor change and can be integrated into the v1.5