Ref T335, misc improvements in value objects

* allow to reset PBH for terrain probe
* check for null altitude before converting
* function isTerrainProbe for model/aircraft

src/blackmisc/simulation/aircraftmodel.h

@@ -277,6 +277,9 @@ namespace BlackMisc
             //! Model type
             ModelType getModelType() const { return m_modelType; }
 
+            //! Terrain probe?
+            bool isTerrainProbe() const { return m_modelType == TypeTerrainProbe; }
+
             //! Model type
             const QString &getModelTypeAsString() const { return modelTypeToString(getModelType()); }