cleanutils simplify step fails when structure hasn't been published

[fatipardo]

The code can be fixed like this:

     if '_citation.title' in mmcif_dict:
         L1 = mmcif_dict['_citation.title']
     else:
         L1 = "???"

     if '_citation.year' in mmcif_dict:
          L2 = mmcif_dict['_citation.year']
     else:
          L2 = "???"

     if '_citation.pdbx_database_id_DOI' in mmcif_dict:
         L3 = mmcif_dict['_citation.pdbx_database_id_DOI']
     else:
         L3 = "???"

[fatipardo]

I need to update cleanutils.py with the new version of the code