fix: predictive_gradients for new posterior class
fix: predictive_gradients for new posterior class

esiivola/GPYgradients,ysekky/GPy,ysekky/GPy,mikecroucher/GPy,esiivola/GPYgradients,SheffieldML/GPy,befelix/GPy,dhhjx880713/GPy,dhhjx880713/GPy,SheffieldML/GPy,ysekky/GPy,mikecroucher/GPy,befelix/GPy,mikecroucher/GPy,mikecroucher/GPy,dhhjx880713/GPy,esiivola/GPYgradients,befelix/GPy,SheffieldML/GPy,ysekky/GPy,esiivola/GPYgradients,befelix/GPy,dhhjx880713/GPy,SheffieldML/GPy