Units should be defined explicitly

[stefanoborini]

After the discussion about units, it is probably important to remember the following: data models generally do not focus on implementing every possible option. What they do is instead choose one standard unit for internal storage, and use that.

Meaning: we don't need a unit ontology, or to express e.g. MASS as an entity that has both a value and a unit. We just need to say that MASS is a value of mass in kilograms. If we want a different unit, it is generally because it is relevant to a specific field (e.g. mass in atomic units is called atomic mass unit and values are expressed as such, e.g. 13 amu)