code: def estimate_position_angle(self,param='position_angle',burn=None,clip=10.0,alpha=0.32):
          
          # 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
docs: Estimate the position angle from the posterior dealing
          with periodicity.