Make EWR sites purchasable.

Fixes https://github.com/dcs-liberation/dcs_liberation/issues/968

gen/sam/airdefensegroupgenerator.py

@@ -21,6 +21,8 @@ class AirDefenseGroupGenerator(GroupGenerator, ABC):
     This is the base for all SAM group generators
     """
 
+    price: int
+
     def __init__(self, game: Game, ground_object: SamGroundObject) -> None:
         ground_object.skynet_capable = True
         super().__init__(game, ground_object)

gen/sam/ewr_group_generator.py

@@ -17,8 +17,8 @@ from gen.sam.ewrs import (
     SnowDriftGenerator,
     StraightFlushGenerator,
     TallRackGenerator,
+    EwrGenerator,
 )
-from gen.sam.group_generator import GroupGenerator
 
 EWR_MAP = {
     "BoxSpringGenerator": BoxSpringGenerator,
@@ -36,7 +36,7 @@ EWR_MAP = {
 
 def get_faction_possible_ewrs_generator(
     faction: Faction,
-) -> List[Type[GroupGenerator]]:
+) -> List[Type[EwrGenerator]]:
     """
     Return the list of possible EWR generators for the given faction
     :param faction: Faction name to search units for

gen/sam/ewrs.py

@@ -5,9 +5,16 @@ from gen.sam.group_generator import GroupGenerator
 
 
 class EwrGenerator(GroupGenerator):
-    @property
-    def unit_type(self) -> VehicleType:
-        raise NotImplementedError
+    unit_type: VehicleType
+
+    @classmethod
+    def name(cls) -> str:
+        return cls.unit_type.name
+
+    @staticmethod
+    def price() -> int:
+        # TODO: Differentiate sites.
+        return 20
 
     def generate(self) -> None:
         self.add_unit(