Leslie Odom Jr. and Freida Pinto make sympathetic, easy-on-the-eyes lovers in “Only,” an absorbing post-catastrophe drama, in theaters and on demand March 6. Consider “Only” a variation on the “What would you do in this horrid situation?” subgenre. Only it’s more a “What would we do?” which can be an exponentially more challenging proposition. (Hard enough to agree on where to get takeout.)