Add fields to DV_QUANTITY for units system and units display name: String.
Values for unit_system field: a URI, see FHIR (https://www.hl7.org/fhir/terminologies-systems.html).
Guidance: these are optional fields; either or both may be set. Unit_label value is for direct display.
If unit system is void, assume UCUM (i.e. that the value is 'http://unitsofmeasure.org')
If unit_label is not filled in, the application system has to convert units to the displayable form itself.