Write the xml data into the file.

        Parameters
        ----------
        filename: Name of the file.

        Examples
        -------
        >>> writer = XMLBIFWriter(model)
        >>> writer.write_xmlbif(test_file)

def write_xmlbif(self, filename):
        """
        Write the xml data into the file.

        Parameters
        ----------
        filename: Name of the file.

        Examples
        -------
        >>> writer = XMLBIFWriter(model)
        >>> writer.write_xmlbif(test_file)
        """
        with open(filename, 'w') as fout:
            fout.write(self.__str__())