I can define a Convertible Unit Category

(User Story, Investigate , Priority: Medium, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Convertible Units
Summary: That is, I can define a category such as "Volume", "Length", "Speed", and so on.
Note that a Category doesn't do much itself -- it's just a kind of Unit. A Unit System then defines one or more Units of this Category.
The Metric system is pre-defined, and serves as the root for converting between heterogeneous Systems.
This is the base story for the Convertible Units Feature.