Spawn unused aircraft at airports.

The aircraft that have not been fragged will now be spawned in parking
to provide more targets for OCA strikes. We do this only at airports,
not carriers. The AI has enough trouble taxiing around uncrowded carrier
decks that we probably shouldn't make it harder for them, plus most of
the aircraft will be stored below the flight deck (we allow 90 aircraft
to be stored at the carrier, which certainly will not fit on the flight
deck).

The aircraft are spawned in an uncontrolled state and nothing will
activate them, so aside from the cost of rendering them they shouldn't
affect performance.

Fixes https://github.com/Khopa/dcs_liberation/issues/148

game/unitmap.py

@@ -19,5 +19,5 @@ class UnitMap:
                 raise RuntimeError(f"Duplicate unit name: {name}")
             self.aircraft[name] = flight
 
-    def flight(self, group_name: str) -> Optional[Flight]:
-        return self.aircraft.get(group_name, None)
+    def flight(self, unit_name: str) -> Optional[Flight]:
+        return self.aircraft.get(unit_name, None)