added rbf kernel compute Hessian
added rbf kernel compute Hessian

SheffieldML/forkedGPmat,SheffieldML/forkedGPmat,lawrennd/GPmat,lawrennd/GPmat,SheffieldML/forkedGPmat,lawrennd/GPmat,SheffieldML/forkedGPmat,SheffieldML/forkedGPmat,SheffieldML/forkedGPmat