Parsing SMILES with heavy atoms #118

Hi there,

First of all, thanks for your work on this package! I make heavy use of it in my mass spectrometry database parser package.
I recently was asked if I could also allow users to provide SMILES structures of heavy labeled (13C) compounds as input. However, i notice that upon parsing SMILES with 13Cs, the string either fails to parse (rarely) or changes the heavy C back to a regular C (mol2formula does not have any heavy atoms included).
Is there any functionality to work with heavy labeled atoms?

Kind regards,
Joanna