Added first draft of login and tour select page

lib/model/receipt.dart

@@ -0,0 +1,18 @@
+import 'package:app_gaslieferung/model/customer.dart';
+
+import 'article.dart';
+
+class Receipt {
+  Customer customer;
+  List<Article> articles;
+
+  double totalGrossPrice;
+  double totalNetPrice;
+
+  Receipt({
+    required this.articles,
+    required this.customer,
+    required this.totalGrossPrice,
+    required this.totalNetPrice,
+  });
+}