update OrderedStop data schema type and test loader method

src/repository.ts

@@ -41,8 +41,8 @@ export interface IEta {
 }
 
 export interface IOrderedStop {
-  nextStopId?: string;
-  previousStopId?: string;
+  nextStop?: IOrderedStop,
+  previousStop?: IOrderedStop,
   routeId: string;
   stopId: string;
 }