m._compute_threshold(z=2.0) -- For Motif objects assembled from a set of sequence,
                                       compute a self.threshold with a z-score based on the distribution
                                       of scores in among the original input sequences.

def _compute_threshold(self,z=2.0):
        """
        m._compute_threshold(z=2.0) -- For Motif objects assembled from a set of sequence,
                                       compute a self.threshold with a z-score based on the distribution
                                       of scores in among the original input sequences.
        """
        scoretally = []
        for seq in self.seqs:
            matches,endpoints,scores = self.scan(seq,-100)
            scoretally.append(scores[0])
        ave,std = avestd(scoretally)
        self.threshold = ave - z *std