Cook County’s embattled state’s attorney asked a judge Thursday to appoint a special prosecutor to take over the case against a white Chicago police officer who shot a black teenager 16 times.Anita M. Alvarez has faced months of criticism over her handling of the case and why it took more than a year to bring charges despite there being police dashcam video of the October 2014 killing of 17-year-old Laquan McDonald.Alvarez, who is set to leave office in December, wrote in a petition to recuse her office from …