Is it possible to have your cake and eat it, too, as long as you burn enough calories to make up for it? In a way, that’s the idea behind the “calories in, calories out” (CICO) diet, a twist on a time-tested principle of weight loss: As long as you burn more calories than you ingest, you will lose weight. But some people are taking that as a license to live on junk food — as long as the math works out. Check out the #CICO hashtag on Instagram an…