The subtlety, cleverness and humanity with which “Sally & Tom” approaches the story of Hemings and Jefferson, dazzlingly doubled in the story of the troupe putting it on, come as no surprise at all. They are the hallmarks of an author incapable of writing a line unfilled with the bewildering burden — or is it the treasure? — of human contradiction. Indeed, Parks begins with an unprovable yet also undisprovable thesis. She has Luce, the author and star of “The Pursuit of Happiness,” decree: “This is not a love story.”