Thanks for the suggestion. I am pretty certain, this unit should be part of
the UCUM (Unified Code for Units of Measure) catalogue, but for reasons
mentioned here at Wikipedia: http://en.wikipedia.org/wiki/SI#Derived_units
the quantity package does not contain a unit typ for "Mass Density".
Tomorrow here in London we have a meeting of the JSR 363 (Unit JSR) EG with
2 Spec Leads including myself and at least one other EG Member present.
While the spec module is likely to be restricted to those in the list above
(or currently in Unit-API 0.6.) extensions and libraries defining additional
quantities may contain such a quantity type.