Add frozen combat modelling.

This doesn't do anything yet, but sets up the data model handling for
frozen combat. The next step is to show combat in the map view, since
that will be helpful when debugging the step after that one: resolving
frozen combat.

This would benefit from caching the Shapely data for SAM threat zones.
Right now it's generating them once per tick and the stuttering is
visible at max speed.

https://github.com/dcs-liberation/dcs_liberation/issues/1680

game/procurement.py

@@ -8,13 +8,13 @@ from typing import Iterator, List, Optional, TYPE_CHECKING, Tuple
 from game import db
 from game.data.groundunitclass import GroundUnitClass
 from game.dcs.groundunittype import GroundUnitType
-from game.factions.faction import Faction
-from game.squadrons import Squadron
 from game.theater import ControlPoint, MissionTarget
 
 if TYPE_CHECKING:
     from game import Game
     from game.ato import FlightType
+    from game.factions.faction import Faction
+    from game.squadrons import Squadron
 
 FRONTLINE_RESERVES_FACTOR = 1.3