code: def covariance_points(self,x0,y0,xother,yother):
          
          dx = x0 - xother
          dy = y0 - yother
          dxx,dyy = self._apply_rotation(dx,dy)
          h = np.sqrt(dxx*dxx + dyy*dyy)
          return self._h_function(h)
docs: get the covariance between base point x0,y0 and
          other points xother,yother implied by Vario2d
  
          Parameters
          ----------
          x0 : (float)
              x-coordinate of base point
          y0 : (float)
              y-coordinate of base point
          xother : (float or numpy.ndarray)
              x-coordinates of other points
          yother : (float or numpy.ndarray)
              y-coordinates of other points
  
          Returns
          -------
          cov : numpy.ndarray
              covariance between base point and other points implied by
              Vario2d.
  
          Note
          ----
          len(cov) = len(xother) = len(yother)