add shipments

lib/domain/entities/cargo.dart

@@ -3,4 +3,15 @@ class Cargo {
   int price;
   int weight;
   Cargo({this.type, this.price, this.weight});
+
+  @override
+  String toString() {
+    return type;
+  }
+
+  @override
+  bool operator ==(Object other) => other is Cargo && other.type == type;
+
+  @override
+  int get hashCode => type.hashCode;
 }