code: def avg_mutual_coverage(self,gpd):
      
      ov = self.overlap_size(gpd)
      if ov == 0: return 0
      xfrac = float(ov) / float(self.get_length())
      yfrac = float(ov) / float(gpd.get_length())
      return sqrt(xfrac*yfrac)
docs: get the coverage fraction of each transcript then return the geometric mean
  
      :param gpd: Another transcript
      :type gpd: Transcript
      :return: avg_coverage
      :rtype: float