THE POT DOCTOR HAS A BACKACHE. As Mahmoud ElSohly, Ph.D. shakes my hand, he’s wincing. Two days ago, ElSohly—the director of the University of Mississippi’s Marijuana Project—bent down the wrong way and threw out his back. And unfortunately, this morning’s visit to his chiropractor didn’t help him much. Ironically, just outside ElSohly’s office in the Waller Complex—behind bolted doors, coded chambers, and security cameras—lies a government-guarded farm where acres of…