added timestamp code, support columns for group by and order by and tested load has-many eager code

include/matador/object/collection.hpp

@@ -8,11 +8,13 @@ namespace matador::object {
 template < class Type >
 class collection {
 public:
-    using value_type = Type;
+  using value_type = Type;
 
+  void push_back(const Type& value) { data_.push_back(value); }
+
+  [[nodiscard]] size_t size() const { return data_.size(); }
 private:
   std::vector<Type> data_;
-
 };
 
 }