KLApproxDiagonalInferenceMethod (the third KL method)

@karlnapf 
Take a look at this.
Note that it may have similar issues you mentioned in other KL methods.
I will correct all of them tomorrow.
Just have it run to ensure everything work well.
Some sample code
```Matlab
clear all; close all;

x1=[0.8822936
-0.7160792
0.9178174
-0.0135544
-0.5275911];

x2=[-0.9597321
0.0231289
0.8284935
0.0023812
-0.7218931];

x=[x1 x2];

y=[1
-1
1
-1
-1];

hyp.cov = log([1; 1]);
hyp.lik=[];
cov = {'covSEiso'};
lik = {@likLogistic}

approxCholeskyWithLBFGS(hyp, cov, lik, x, y)
```