Add area density files to materials schema

[DsposalTom]

Paper and plastic seem to really require an area density value and unit of measure. To fit in with other values throughout the standard we should create

areaDensity - Recommended - Numeric - The area density of the material. Where area density is the measure of how much mass is packed into a given area of a two-dimensional object. Provided in grams per square metre (gsm).
areaDensityTolerance - Recommended - Numeric - The threshold of area density that the material can vary by. This is given as a +/- value.
areaDensityToleranceType - Recommended - String - Either gsm or percentage based on the value provided in areaDensityTolerance
areaDenistyDate - Recommended - String - The date that the area density was last verified/measured. Use the format dd/mm/yyyy.

[DsposalTom]

Weights and Measures

Within Open 3P there are very few weights and measurements however working with organisations it feels like this needs to be expandable on at various levels of the packaging value chain.
The main consideration is around mass, density, thickness, area density and yield.

Mass

The mass of the material. Where mass is a fundamental property of an object that represents the amount of matter it contains. Provide in grams (g).

Density

The density of the material. Where density is the mass of a unit volume of a material substance. The formula for density is d = M/V where d is density, M is mass, and V is volume. Provide in grams per cubic centimetre (g/ cm3).

Thickness

The thickness of the material. Where thickness is the distance from one side of the material to the opposite side. For example, the thickness of a cardboard or the thickness of glass. Provide in micrometre (μm), micron.

Area Density

The area density of the material. Where area density is the measure of how much mass is packed into a given area of a two-dimensional object. Provide in grams per square metre (g/ m²), GSM.

Yield

The yield of the material. Where yield is a measure of the material’s coverage per unit of mass. Provide in square meter per kilogram (m²/ kg).

[DsposalTom]

Weights/ measures