Compute double integral of the covariance kernel over a [0,1]^d domain
Wij(mu1, mu2 = NULL, theta, type)input locations considered
lengthscale hyperparameter of the kernel
kernel type, one of "Gaussian", "Matern5_2" or "Matern3_2", see cov_gen
M. Binois, J. Huang, R. Gramacy, M. Ludkovski (2017+), Replication or exploration? Sequential design for stochastic simulation experiments.