Added first draft of login and tour select page

lib/model/article.dart

@@ -0,0 +1,31 @@
+class Article {
+  String articleId;
+
+  /// The quantity of delivered gas cylinders.
+  double quantity;
+
+  /// The price per cylinder in euros.
+  double pricePerQuantity;
+
+  /// The price of the deposit in euros per cylinder.
+  double depositPricePerQuantity;
+
+  /// The amount of cylinders delivered.
+  double quantityDelivered;
+
+  /// The amount of cylinders returned by the customer.
+  double quantityReturned;
+
+  /// The amount of cylinders left to be deposited.
+  double quantityToDeposit;
+
+  Article({
+    required this.articleId,
+    required this.quantity,
+    required this.pricePerQuantity,
+    required this.depositPricePerQuantity,
+    required this.quantityDelivered,
+    required this.quantityReturned,
+    required this.quantityToDeposit,
+  });
+}