Add missing "addTrain" operation

src/main/java/fr/univnantes/trainreservation/TicketReservationSystem.java

@@ -81,6 +81,12 @@ public interface TicketReservationSystem {
      */
     void addCity(City city);
 
+    /**
+     * Adds a new train in the system.
+     * @param train The train to add.
+     */
+    void addTrain(Train train);
+    
     /**
      * Creates and registers a new trip in the system.
      * This new trip must take place *after* the last trip of the chosen train (based on the real arrival time).

src/main/java/fr/univnantes/trainreservation/impl/TicketReservationSystemImpl.java

@@ -164,4 +164,10 @@ public class TicketReservationSystemImpl implements TicketReservationSystem {
     public List<Trip> findOrderedTripsOfTrain(Train train) {
         return trips.stream().sorted((t1, t2) -> t1.findRealArrivalTime().isBefore(t2.findRealDepartureTime()) ? -1 : 1).collect(Collectors.toList());
     }
+
+    @Override
+    public void addTrain(Train train) {
+        this.trains.add(train);
+        
+    }
 }