add method to check if notification is scheduled

src/repositories/InMemoryNotificationRepository.ts

@@ -42,6 +42,10 @@ export class InMemoryNotificationRepository implements NotificationRepository {
     return this.deviceIdsToDeliverTo[tuple.toString()][deviceId];
   }
 
+  async isNotificationScheduled(lookupArguments: NotificationLookupArguments): Promise<boolean> {
+    return await this.getSecondsThresholdForNotificationIfExists(lookupArguments) !== null;
+  }
+
   async addOrUpdateNotification({
     shuttleId,
     stopId,
@@ -71,4 +75,5 @@ export class InMemoryNotificationRepository implements NotificationRepository {
 
     delete this.deviceIdsToDeliverTo[tupleKey.toString()][deviceId];
   }
+
 }