The unveiling of four new nonstop international flights from Denver International Airport, including a first direct flight to Rome, has boosted DIA’s emergence as a global hub with international air traffic surging more than twice as fast as domestic travel. Aeromexico on Saturday will announce nonstop flights from DIA to Monterrey and Guadalajara, airport officials said, and United Airlines officials on Thursday unveiled the 5,563-mile flight to Rome, starting May 1, along with a flight to Regina, Canada. Travelers at DIA now can fly nonstop to 33 destinations in 18 countries.