The base KL method class

@karlnapf 
Take a look at this base class.
Once it is ready to merge, I will send the two sub-class KL methods.
I think it is listed in the beginning 
```
 * Code adapted from 
 * http://hannes.nickisch.org/code/approxXX.tar.gz
 * and Gaussian Process Machine Learning Toolbox
 * http://www.gaussianprocess.org/gpml/code/matlab/doc/
 * and the reference paper is
 * Nickisch, Hannes, and Carl Edward Rasmussen.
 * "Approximations for Binary Gaussian Process Classification."
 * Journal of Machine Learning Research 9.10 (2008).
```