code: def covariance(self,pt0,pt1):
          
          #raise Exception()
          cov = self.nugget
          for vario in self.variograms:
              cov += vario.covariance(pt0,pt1)
          return cov
docs: get the covariance between two points implied by the GeoStruct.
          This is used during the ordinary kriging process to get the RHS
  
          Parameters
          ----------
          pt0 : (iterable length 2 of floats)
          pt1 : (iterable length 2 of floats)
  
          Returns
          -------
          covariance : float
              the covariance between pt0 and pt1 implied by the GeoStruct