Compute the diagonal of the covariance matrix associated to X.

def Kdiag(self, X, target):
        """Compute the diagonal of the covariance matrix associated to X."""
        ly=1/self.lengthscaleY
        lu=np.sqrt(3)/self.lengthscaleU
        #ly=self.lengthscaleY
        #lu=self.lengthscaleU
        
        k1 = (2*lu+ly)/(lu+ly)**2
        k2 = (ly-2*lu + 2*lu-ly ) / (ly-lu)**2 
        k3 = 1/(lu+ly) + (lu)/(lu+ly)**2 

        np.add(self.varianceU*self.varianceY*(k1+k2+k3), target, target)