The U.S. has the largest concentration of billionaires and millionaires in the world, according to the latest data, but the ultrarich are increasingly looking elsewhere for better, safer opportunities. Forty percent of all the global ultrarich, some 221,800 individuals, call the U.S. their home, according to a new report by the financial advisory firm Arton Capital, making the country the world's dominant wealth hub. Of these, an estimated 205,000 are primary residents in the country. But