derived from CStreamable, for streaming to/from QtDBusArgument, avoiding
the unrelated operator function template for streaming Container<T>
defined by Qt, by being a better match in overload resolution.
* 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
