docs: def add_sdfile(self, sdfile):
          """Add new ``SDfile`` to current ``SDfile``.
  
          :param sdfile: ``SDfile`` instance. 
          :return: None.
          :rtype: :py:obj:`None`.
          """
          if not isinstance(sdfile, SDfile):
              raise ValueError('Not a SDfile type: "{}"'.format(type(sdfile)))
  
          for entry_id in sdfile:
              self.add_molfile(molfile=sdfile[entry_id]['molfile'],
                               data=sdfile[entry_id]['data'])