DULUTH — Less than a year after taking over a once-idled mill, the Sofidel Group, an Italian multinational papermaker, hopes to grow its local operations substantially. Sofidel aims to invest at least $180 million in its new Duluth mill and probably closer to $250 million when all is said and done, said Chad Ronchetti, director of Duluth’s planning and economic development division. That expansion promises to bring another 160 jobs to Dulut…