fix issue #381. (#383)

The returned system should be of the same type as the perturbed system

Co-authored-by: Han Wang <[email protected]>

Author: amcadmus

dpdata/system.py

@@ -810,6 +810,11 @@ def perturb(self,
         perturbed_system : System
             The perturbed_system. It contains `pert_num` * frame_num of the input system frames.
         """
+        if type(self) is not dpdata.System:
+            raise RuntimeError(
+                f'Using method perturb() of an instance of {type(self)}. '
+                f'Must use method perturb() of the instance of class dpdata.System.'
+            )
         perturbed_system = System()
         nframes = self.get_nframes()
         for ii in range(nframes):