Change squadrons to operate out of a single base.

https://github.com/dcs-liberation/dcs_liberation/issues/1145

Currently this is fixed at the start of the campaign. The squadron
locations are defined by the campaign file.

Follow up work:

* Track aircraft ownership per-squadron rather than per-airbase.
* UI for relocating squadrons.
* Ferry missions for squadrons that are relocating.
* Auto-relocation (probably only for retreat handling).

Fixes https://github.com/dcs-liberation/dcs_liberation/issues/1138

game/theater/controlpoint.py

@@ -54,6 +54,7 @@ from ..weather import Conditions
 if TYPE_CHECKING:
     from game import Game
     from gen.flights.flight import FlightType
+    from game.squadrons.squadron import Squadron
     from ..transfers import PendingTransfers
 
 FREE_FRONTLINE_UNIT_SUPPLY: int = 15
@@ -322,6 +323,8 @@ class ControlPoint(MissionTarget, ABC):
 
         self.target_position: Optional[Point] = None
 
+        self.squadrons: list[Squadron] = []
+
     def __repr__(self) -> str:
         return f"<{self.__class__}: {self.name}>"