set size_threshold default to zero

Author: donerancl

rmgpy/molecule/fragment.py

@@ -836,7 +836,7 @@ def from_rdkit_mol(self, rdkitmol, atom_replace_dict=None):
 
         return self
 
-    def cut_molecule(self, output_smiles=False, cut_through=True, size_threshold=None):
+    def cut_molecule(self, output_smiles=False, cut_through=True, size_threshold=5):
         """
         For given input, output a list of cut fragments (either string or Fragment).
         if output_smiles = True, the output list of fragments will be smiles.