minor fix

pina/problem/timedep_problem.py

@@ -5,7 +5,10 @@ from .abstract_problem import AbstractProblem
 
 class TimeDependentProblem(AbstractProblem):
 
-    @property
     @abstractmethod
-    def temporal_variable(self):
+    def temporal_domain(self):
         pass
+
+    @property
+    def temporal_variables(self):
+        return self.temporal_domain.variables