Update system.py

Author: njzjz

dpdata/system.py

@@ -1256,7 +1256,7 @@ def to_deepmd_npy(self, folder, set_size = 5000, prec=np.float32) :
 def check_System(data):
     keys={'atom_names','atom_numbs','cells','coords','orig','atom_types'}
     assert( isinstance(data,dict) )
-    assert( set(data.keys())==keys )
+    assert( keys.issubset(set(data.keys())) )
     if len(data['coords']) > 0 :
         assert( len(data['coords'][0])==len(data['atom_types'])==sum(data['atom_numbs']) )
     else :