Write segments to file.

            - filename, (str) location of output file

def _write_segments(self, filename):
        """ Write segments to file.

            - filename, (str) location of output file
        """
        # Column headers
        shead = '\t'.join(['id', 'multiplicon', 'genome', 'list', 'first',
                           'last', 'order'])
        with open(filename, 'w') as fhandle:
            fhandle.write(shead + '\n')
            for mrow in self.segments:
                fhandle.write('\t'.join([str(e) for e in mrow]) + '\n')