Germany’s president criticized what he called a “thoughtless, frivolous” approach by some artists toward Israel as he opened the sprawling documenta exhibition, a three-month spectacle which puts the city of Kassel on the art world map every five years. “Art is allowed to be offensive, it should trigger debate. But the line is crossed when criticism of Israel turns into questioning its existence,” President Frank-Walter Steinmeier said as the 15th edition of the contemporary art festival got under way in the central German city on Saturday. He said that the recognition of Israel as a nation is…