Add ``Molfile`` and data to ``SDfile`` object.

        :param molfile: ``Molfile`` instance.
        :type molfile: :class:`~ctfile.ctfile.Molfile`.
        :param dict data: Data associated with ``Molfile`` instance. 
        :return: None.
        :rtype: :py:obj:`None`.

def add_molfile(self, molfile, data):
        """Add ``Molfile`` and data to ``SDfile`` object.

        :param molfile: ``Molfile`` instance.
        :type molfile: :class:`~ctfile.ctfile.Molfile`.
        :param dict data: Data associated with ``Molfile`` instance. 
        :return: None.
        :rtype: :py:obj:`None`.
        """
        if not isinstance(molfile, Molfile):
            raise ValueError('Not a Molfile type: "{}"'.format(type(molfile)))

        if not isinstance(data, dict):
            raise ValueError('Not a dict type: "{}"'.format(type(data)))

        entry_ids = sorted(self.keys(), key=lambda x: int(x))
        if entry_ids:
            last_entry_id = str(entry_ids[-1])
        else:
            last_entry_id = '0'

        new_entry_id = str(int(last_entry_id) + 1)
        self[new_entry_id] = OrderedDict()
        self[new_entry_id]['molfile'] = molfile
        self[new_entry_id]['data'] = data