Allan Kronzek, a professional magician who lives in Sag Harbor, has pulled off a wonderful trick – writing a how-to book for grandparents that’s designed to connect them with their grandkids. Called “Grandpa Magic,” but intended also for Grandmas, the book declares that its 116 amazing brain teasers, perplexing puzzles and simple stunts will wow kids of all ages. And lure them away from their digital devices for a while. Kronzek hopes to engage kids in an imaginative world full of mystery,