Fix sending the correct pressure altitude for a pilot position

* Before this commit, only the true altitude was known for an aircraft situation. The pressure altitude was not available anywhere yet.
* This caused a wrong altitude in radar clients.
* We fix this reading the pressure altitude from the simulators and set it in the own aircraft situation.
* MS Flight Simulators have the pressure altitude in the APIs available.
* For X-Plane and emulated simulator, we need to calculate it from the pressure at sea level.
* Finally, we use the new available pressure altitude to send it to the FSD server.

Maniphest Tasks: Ref T223

tests/blackmisc/testaviation.h

@@ -47,6 +47,9 @@ namespace BlackMiscTest
 
         //! Testing copying and equality of objects
         void copyAndEqual();
+
+        //! CAltitude tests
+        void altitude();
     };
 } // namespace