Add new methods to RootCoordinator: tn getter

src/artis-star/common/RootCoordinator.hpp

@@ -122,6 +122,8 @@ public :
 
   void switch_to_timed_observer(double step) { _observer.switch_to_timed_observer(step); }
 
+  typename Time::type tn() const { return _tn; }
+
   std::string to_string() const {
     std::ostringstream ss;