Adding mass or volume to productIngredientType

[donwilson-rezare]

In issue #85 it was proposed to add "percentage" to productIngredientType.

An additional question is whether mass and volume and their associated units should also be added to productIngredientType.

This would allow the mass or volume of an active ingredient to be specified for a product.

[cookeac]

If we do this, we would need to allow for a demoninator unit as well (either by explicitly having a denominator, or by choosing UNCEFACT units with a denominator). For example "Abamectin 1 mg/mL".