support list for System indexing (#264)

Author: njzjz

dpdata/system.py

@@ -162,7 +162,7 @@ def __str__(self):
 
     def __getitem__(self, key):
         """Returns proerty stored in System by key or by idx"""
-        if isinstance(key, (int, slice)):
+        if isinstance(key, (int, slice, list)):
             return self.sub_system(key)
         return self.data[key]