deriv_2nd_ar1(phi, sig2, tau)
double
corresponding to the phi coefficient of an AR(1) process.double
corresponding to the error term of an AR(1) process.vec
that contains the scales to be processed (e.g. 2^(1:J))matrix
with the first column containing the second partial derivative with respect to $sigma^2$ and the second column contains the second partial derivative with respect to $sigma^2$