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