accept np.ndarray as index (#407)

Author: njzjz

dpdata/system.py

@@ -339,7 +339,7 @@ def __str__(self):
 
     def __getitem__(self, key):
         """Returns proerty stored in System by key or by idx"""
-        if isinstance(key, (int, slice, list)):
+        if isinstance(key, (int, slice, list, np.ndarray)):
             return self.sub_system(key)
         return self.data[key]