A new trend is spreading. Well, a new trend that involves a very old pastime.  Introducing: the “churn and burn,” a viral challenge that gives all new meaning to the phrase “grocery run.” It started when a couple of Oregon-based trail runners decided to test out how easily they could make butter in their vests during a run. The answer, they discovered, is that it’s actually a pretty smooth process. It was earlier this year that Libby Cope and her boyfriend Jacob Arnold, both 30, were first