Return name of file containing the alignment

        prefix -- str, prefix of alignment file.

def _tree_filename(self):
        """Return name of file containing the alignment

        prefix -- str, prefix of alignment file.
        """
        if self.Parameters['--out']:
            aln_filename = self._absolute(self.Parameters['--out'].Value)
        else:
            raise ValueError, "No tree output file specified."
        return aln_filename