Meet John and Barbara Stephens-Lewallen: They harvest seaweed. Operators of the Mendocino Sea Vegetable Company in Philo, California, the Stephens-Lewallens are farmers/fishermen of another stripe. Their catch includes bladderwrack, sea lettuce, kombu, and nori.   The couple reveals the culinary delights of seaweed tea and fried seaweed. And Chef Eric Tucker of Millennium Restaurant demonstrates what someone can do with a little sea palm.