refs #865, hints get ground elevation from provider or from set value

* using CAltitude for elevation provider
* null() for CAltitude
* formatting of members
* fixme in fs9 client

src/blackmisc/aviation/altitude.cpp

@@ -106,5 +106,11 @@ namespace BlackMisc
             return BlackMisc::CIcon::iconByIndex(CIcons::GeoPosition);
         }
 
+        const CAltitude &CAltitude::null()
+        {
+            static const CAltitude null(0, CAltitude::MeanSeaLevel, CLengthUnit::nullUnit());
+            return null;
+        }
+
     } // namespace
 } // namespace

src/blackmisc/aviation/altitude.h

@@ -63,7 +63,7 @@ namespace BlackMisc
             /*!
              * Enum type to distinguish between MSL and AGL
              */
-            enum ReferenceDatum : uint
+            enum ReferenceDatum
             {
                 MeanSeaLevel = 0,   //!< MSL
                 AboveGround,        //!< AGL
@@ -115,6 +115,9 @@ namespace BlackMisc
             //! \copydoc BlackMisc::Mixin::Icon::toIcon
             BlackMisc::CIcon toIcon() const;
 
+            //! Null altitude (MSL)
+            static const CAltitude &null();
+
         private:
             ReferenceDatum m_datum; //!< MSL or AGL?