Added basic tests for Qty.getPrefixes, Qty.getKinds and Qty.getUnits.
Implemented Qty.getPrefixes, Qty.getKinds and Qty.getUnits.
Wrote missing documentation for Qty.getUnits.
Removed Qty.getPrefixes and Qty.getKinds because they are pretty much redundant by Qty.getUnits' dictionary structure.