The new compare is implemented using "multimethods" described in the book Advanced C++ Programming Styles and Idioms by James Coplien.
First, the isA method is used to determine which of the values being compared is the most general. (For example, CLength is more general than CAltitude.)
Then the compareImpl method is called on the most general value, with the other value as an argument.
If there is not a direct inheritance relation between the two values (or they are the same class) then the comparison is invalid and a assert is triggered.
Some interesting results:
* Minor simplification of PQ converters and unit ctors; may not seem like much, but I think it enables possible future simplifications of a similar kind.
* CMeasurementPrefix is gone, no longer needed. Now prefixes are applied at the template stage.
* Apart from that, PQs public API is unchanged.
* Discovered that ft/s^2 conversion factor was wrong: fixed.
* Default unit is more clearly stated in one place, not restated in many different places, and is not always the SI unit
* Converter strategy pattern in CMeasurementUnit, covering linear, affine, and different kinds of sexagesimal units
* General reorganization of CMeasurementUnit construction and CPhysicalQuantity methods, not removing any behvaiour
* Move duplicated method unitFromSymbol from derived classes into base class CMeasurementUnit
* For DBus, CPhysicalQuantity marshals both in its own unit and in the default unit
