“Dinosaur 13” is “Kramer vs. Kramer” for paleontologists, an absorbing documentary that recounts a torturous decade-long custody battle over the miraculously well-preserved skeletal leftovers of a Tyrannosaurus Rex that were discovered along a remote stretch of the South Dakota prairie. As in the 1979 best-picture winner about a divorced father who fights the judicial system to keep his son, there is a heart-rending relationship at the core of “Dinosaur 13” that is initially one of joy and triumph before it becomes mired in perplexing legalities, grandstanding politics, hurtful accusations and unfair repercussions. Advertisement Instead of a boy named Billy, there is a T. Rex named Sue–so dubbed in honor