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/TrainExecution.ts

@@ -22,7 +22,7 @@ export class TrainExecution implements Execution {
   private usedTiles: TileRef[] = []; // used for cars behind
   private stations: TrainStation[] = [];
   private currentRailroad: OrientedRailroad | null = null;
-  private speed: number = 3;
+  private speed: number = 2;
 
   constructor(
     private railNetwork: RailNetwork,
@@ -30,9 +30,7 @@ export class TrainExecution implements Execution {
     private source: TrainStation,
     private destination: TrainStation,
     private numCars: number,
-  ) {
-    this.hasCargo = source.unit.type() === UnitType.Factory;
-  }
+  ) {}
 
   init(mg: Game, ticks: number): void {
     this.mg = mg;