Some cruise lines try to be all things to all people. Viking isn't one of them.Since its founding in 1997, the upscale cruise brand has carved out a niche catering to a certain type of thoughtful, inquisitive, generally older traveler looking to explore the world and learn a thing or two along the way.Most Viking customers are in their retirement years — or quickly approaching retirement — and they're eager to finally see all the places they did…