renaming method name

gem_stanley_teacher.py

@@ -92,7 +92,7 @@ class DTreeGEMStanleyGurobiTeacher(DTreeGurobiTeacherBase):
         super().__init__(name=name,
                          state_dim=3, perc_dim=2, ctrl_dim=1, norm_ord=norm_ord)
 
-    def is_positive_example(self, ex) -> bool:
+    def is_safe_state(self, ex) -> bool:
         assert len(ex) == self.state_dim + self.perc_dim
 
         def g(cte, psi):