Daniel Porterfield, who as president of 2,200-student Franklin & Marshall College (PA) championed the enrollment of low-income students, said Thursday that he will step down next year to lead a Washington-based think tank.  Porterfield, 56, the liberal arts college’s 15th president, has led the private institution since March 2011. He plans to stay through the academic year before taking over as president and CEO of the Aspen Institute on June 1.