Add to schema- preferred unit of measure

I was reading a [paper](https://link.springer.com/article/10.1007/s11367-013-0662-5) on LCA communication and came across an interesting note on use of units-
" Avoid inventing units: Despite the provision in the guide for authors to use SI-units, many articles in this journal still use weird units, such as “DALY (Disability-Adjusted Life Year),” “milli-point (mPt),” and “Potentially affected fraction of species (PAF).” Of course, the SI is not complete; for instance, it lacks monetary units such as dollar or yen. So from time to time, it may be needed to introduce additional units outside SI. But for the given examples, this is not needed. “DALY” is just a year, and the other two are dimensionless. "

I remembered we discussed about having a provision for inserting any type of unit (even those not covered by the OM ontology). However, with respect to the prefered unit perhaps we should specify that only SI-units should be used (your opinions? )