feat(departures): make some adaptations to supp diff departure srces

with these adaptations to the api_handler and Departure.fromJson,
the fromJson-factory now works for responses from both /departureBoard
and /tripDetail

for /tripDetail a little json-copying is used, see
ApiHandler._expandDeparturesJson

lib/transport_helper.dart

@@ -824,16 +824,32 @@ class Departure {
     );
   }
 
+  // this function is built to construct from different json sources
+  // for our hafas api, that means requests to both /departureBoard and /tripDetail
+  // sources from departureBoard were the default, but with some adaptations,
+  // tripDetail is a working source too, with the following workarounds:
+  //
+  // tripDetail has stops as list in a sublevel, whereas "ref", and "Product"
+  // are in the top Level.
+  // stop/station infos on the other hand just use different keys
+  // Therefore this function uses some catches to serve both cases and needs to
+  // be served a somewhat expanded json Map, as seen in
+  // ApiHander._expandDeparturesJson, which copies the topLevel fields into the
+  // repeated "Stop" entries that this function takes as json param
   factory Departure.fromJson(Map<String, dynamic> json) {
     Vehicle vehicle = Vehicle.fromJson(json["Product"][0]);
     vehicle.dbInsert();
     Station station = Station(
-      id: json["stopid"],
-      name: json["stop"],
+      id: json["stopid"] ?? json["id"],
+      name: json["stop"] ?? json["name"],
       transportLines: [],
     );
     station.dbInsert();
 
+    // sometimes no arrivalTime exists, e.g. on the first station in a list
+    // and sometimes no departureTime, as for the last station...
+    // we might in these cases not need to ask for them, but I like a little
+    // redundancy more than handling null values
     DateTime? arrDateTime;
     String? arrivalDate = json["arrDate"];
     String? arrivalTime = json["arrTime"];
@@ -849,7 +865,10 @@ class Departure {
     }
 
     return Departure(
-      ref: json["JourneyDetailRef"]["ref"],
+      // asking for tripDetail version first because
+      // ["JourneyDetailRef"]["ref"] cannot be caught if ["JourneyDetailRef"]
+      // is non-existent
+      ref: json["ref"] ?? json["JourneyDetailRef"]["ref"] as String,
       vehicleId: vehicle.id,
       vehicleName: vehicle.name,
       stationId: station.id,
@@ -979,4 +998,4 @@ enum VehicleType {
   subway,
   regionalTrain,
   PLACEHOLDER,
-}
+}