builder progress (attribute, object and constraint class in todo.md)

include/matador/object/attribute.hpp

@@ -93,6 +93,23 @@ private:
   std::shared_ptr<attribute> reference_column_;
 };
 
+
+class object {
+public:
+    using iterator = std::vector<attribute>::iterator;
+    using const_iterator = std::vector<attribute>::const_iterator;
+
+private:
+    std::vector<attribute> columns_;
+    std::string name_;
+    std::string alias_;
+};
+
+class constraint {
+public:
+private:
+    std::shared_ptr<attribute> column_;
+};
 /**
  * User defined literal to have a shortcut creating a column object
  * @param name Name of the column