Cholesky kl method (the second KL method)

@karlnapf 
The same at here. I use log_det in MatrixOperations.cpp 
The reference result is generated from the Matlab code I modified based on the provided code.

I will soon post my Matlab code in my Github and send a link.

Basically, I tested my Matlab code in some small dataset and compared the result with GPML 3.4 and the provided Matlab code to ensure the correctness of my Matlab codes.

Let me know your suggestion.