Increase accuracy of gradient search and check parameters in unit test

- checked parameters of CExactInferenceMethod (~50 gradient search steps vs ~550 in GPML)
- parameters of EP didn't checked, since there are a very huge difference between different calls.

For example, best hyparameters from GPML for EP with the same initial data:

* first call:

width =  10.7350381445812
scale =  135.740816464898

* the second one:

width =  10.7333023367444
scale =  79.3076727325428