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/api_handler.dart

@@ -218,10 +218,30 @@ class ApiHandler {
         json.decode(jsonResponse.body) as Map<String, dynamic>;
     final DepartureList = httpPackageJson["Stops"]["Stop"]
         .map(
-          (x) => Departure.fromJson(x),
+          (x) => Departure.fromJson(
+            // needed because I'm too lazy to write a second mostly identical
+            // constructor and a wrapper
+            _expandDeparturesJson(httpPackageJson, x, ["ref", "Product"]),
+          ),
         )
         .toList()
         .cast<Departure>();
     return DepartureList;
   }
-}
+
+  // workaround for the fact that json from tripDetail and DepartureBoard are
+  // quite different from each other in their structure... but I don't wish to
+  // build a second constructor >:(
+  // so this function duplicates topLevel Maps into a lower level to be used in
+  // mapping functions and such
+  static Map<String, dynamic> _expandDeparturesJson(
+    Map<String, dynamic> topLevelJson,
+    Map<String, dynamic> subLevelJson,
+    List<String> topLevelKeys,
+  ) {
+    for (final String entry in topLevelKeys) {
+      subLevelJson[entry] = topLevelJson[entry];
+    }
+    return subLevelJson;
+  }
+}