As Abraham Lincoln, David Selby has the weight of the union on his shoulders in “The Heavens Are Hung in Black.” Photo by T. Charles Erickson. In the handy study guide that accompanies the world premiere production of The Heavens Are Hung in Black, now running at the newly reopened Ford’s Theatre, playwright James Still observes that his subject, Abraham Lincoln, is “probably the most written-about person in the world after Jesus.” If that’s…