added observer to repository and updated tests

source/core/object/observer.cpp

@@ -0,0 +1,10 @@
+#include "matador/object/observer.hpp"
+
+namespace matador::object {
+abstract_observer::abstract_observer(const std::type_index& ti)
+: type_index_(ti) {}
+
+const std::type_index& abstract_observer::index() const {
+  return type_index_;
+}
+}  // namespace matador::object

source/core/object/repository_node.cpp

@@ -55,7 +55,11 @@ const repository& repository_node::schema() const {
 bool repository_node::has_children() const {
   return first_child_->next_sibling_ != last_child_;
 }
+void repository_node::on_attach() const {
+  info_->on_attach();
+}
 
+void repository_node::on_detach() const {}
 repository_node::node_ptr repository_node::next() const {
   // if we have a child, child is the next iterator to return
   // (if we don't iterate over the siblings)