Robert Dallek, one of America’s most respected authorities on the presidency and modern American history, has been named the Gen. Mark. Clark Chair of History for the spring semester.   As a visiting professor, Dallek will teach courses on the American presidency. He also will address the South Carolina Corps of Cadets at 6:50 p.m. Tuesday, Jan. 18, in McAlister Field House. The lecture “John Fitzgerald Kennedy: Fifty Years After" is free and open to the public.