A new PBS documentary, “The Kennedy Half-Century,” directed by local filmmaker Paul Roberts for WCVE Richmond, highlights the impact of the 35th president on the nation’s political culture and on the nine presidents who followed him. The film, which accompanies the book of the same title by Larry Sabato, head of the Center for Politics at the University of Virginia, will screen at the Library of Virginia in Richmond on Tuesday before going on nationwide television rotation in the weeks leading up to the 50th anniversary of the assassination on Nov. 22.