Dec. 22—Dartmouth was knocking on Isaac Ahn's door. Columbia, too. In fact, nearly every Ivy League university took an interest in Ahn the student (a National Merit Scholarship semifinalist, hoping to advance as a finalist); Ahn the person (he's the co-captain of Mayo's chess team, among other interests) and Ahn the athlete (a two-time Minnesota Class 3A high school state meet qualifier). Ahn …