Return an empty string of the abbreviation is null
Unit.equals() is based on the resource URI string
More units
Added another unit test; more of the same
Be more verbose in the error message
Added a test class for time units
Ignore another non-parent unit type
Check that converting with ops:Molar works
Added a test for instantiating unit classes using the Unit Ontology (planned)
Working factory for units based on the Unit Ontology
More mapping important to Open PHACTS