Fernando Alonso won the world title in 2005 and 2006Fernando Alonso will be "challenging" for Aston Martin when he joins them next year, says team boss Mike Krack.The two-time champion is joining Aston Martin from Alpine, who would not offer him the terms he wanted to stay on.Krack said: "He can push us, more than a driver who does not have that calibre. It will maybe be much more difficult than it is now."We discussed this. We said: 'What are the pros and cons?' And we came to the conclusion that it is the right step."Alonso is a legendary figure…