Automatic train stations (#1353)

## Description:

Train stations are now built automatically when a factory is
constructed.

Changes:
- When a factory is built, nearby structures are connected to the rail
network
- When a city is built near a factory, it is connected to the rail
network
    - All structures behave the same when a train stops: to be defined
    - Removed station badge
    - Gold income is now related to the structure's level

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

src/core/execution/FactoryExecution.ts

@@ -1,28 +1,30 @@
 import { Execution, Game, Player, Unit, UnitType } from "../game/Game";
 import { TileRef } from "../game/GameMap";
+import { TrainStationExecution } from "./TrainStationExecution";
 
 export class FactoryExecution implements Execution {
   private factory: Unit | null = null;
   private active: boolean = true;
-
+  private game: Game;
   constructor(
     private player: Player,
     private tile: TileRef,
   ) {}
 
   init(mg: Game, ticks: number): void {
-    const spawnTile = this.player.canBuild(UnitType.Factory, this.tile);
-    if (spawnTile === false) {
-      console.warn("cannot build factory");
-      this.active = false;
-      return;
-    }
-    this.factory = this.player.buildUnit(UnitType.Factory, spawnTile, {});
+    this.game = mg;
   }
 
   tick(ticks: number): void {
-    if (this.factory === null) {
-      throw new Error("Not initialized");
+    if (!this.factory) {
+      const spawnTile = this.player.canBuild(UnitType.Factory, this.tile);
+      if (spawnTile === false) {
+        console.warn("cannot build factory");
+        this.active = false;
+        return;
+      }
+      this.factory = this.player.buildUnit(UnitType.Factory, spawnTile, {});
+      this.createStation();
     }
     if (!this.factory.isActive()) {
       this.active = false;
@@ -41,4 +43,21 @@ export class FactoryExecution implements Execution {
   activeDuringSpawnPhase(): boolean {
     return false;
   }
+
+  createStation(): void {
+    if (this.factory !== null) {
+      const structures = this.game.nearbyUnits(
+        this.factory.tile()!,
+        this.game.config().trainStationMaxRange(),
+        [UnitType.City, UnitType.Port, UnitType.Factory],
+      );
+      // Use different seeds or trains will spawn simultaneously
+      let seed = 0;
+      for (const { unit } of structures) {
+        if (!unit.hasTrainStation()) {
+          this.game.addExecution(new TrainStationExecution(unit, ++seed));
+        }
+      }
+    }
+  }
 }