Function for computing the fixed variance kernel.
fixedvarianceKernCompute(kern, x, x2)GP kernel structure which contains the
fixed variances to be included in the fixedvariance
kernel.
One-column matrix which contains the input values for the rows of the kernel.
One-column matrix which contains the input
values for the columns of the kernel. If missing,
a square matrix is created with x on both
rows and columns.
Return kernel with the fixed variances in the diagonal, and zeros in the off-diagonal elements.