Ref T223, use const values for standard pressure

* standardISASeaLevelPressure
* use static const values for standard pressure
* minor formatting

src/blackmisc/aviation/altitude.h

@@ -168,6 +168,9 @@ namespace BlackMisc
             //! Null altitude (MSL)
             static const CAltitude &null();
 
+            //! Standard pressure 1013.25mbar/hPa
+            static const PhysicalQuantities::CPressure &standardISASeaLevelPressure();
+
         private:
             ReferenceDatum m_datum; //!< MSL or AGL?
             AltitudeType m_altitudeType = TrueAltitude;