Estimate the position angle from the posterior dealing
        with periodicity.

def estimate_position_angle(self,param='position_angle',burn=None,clip=10.0,alpha=0.32):
        """ Estimate the position angle from the posterior dealing
        with periodicity.
        """
        # Transform so peak in the middle of the distribution
        pa = self.samples.get(param,burn=burn,clip=clip)
        peak = ugali.utils.stats.kde_peak(pa)
        shift = 180.*((pa+90-peak)>180)
        pa -= shift
        # Get the kde interval
        ret = ugali.utils.stats.peak_interval(pa,alpha)
        if ret[0] < 0: 
            ret[0] += 180.; ret[1][0] += 180.; ret[1][1] += 180.;
        return ret