First major wave of PQ refactoring, including but not limited to:

* 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

src/blackmisc/pqconstants.h

@@ -69,6 +69,16 @@ public:
         return p;
     }
 
+    /*!
+     * \brief Standard pressure datum for flight levels expressed in mmHg, such as in Russia, 760mmHg
+     * \return
+     */
+    static const CPressure& RuFlightLevelPressure()
+    {
+        static CPressure p(760, CPressureUnit::mmHg());
+        return p;
+    }
+
     /*!
      * \brief Unicom frequency
      * \return