We’re nearing the end of a dreary Chicago winter — a time when many people admit to being in a fitness slump. It’s hard to find motivation to workout when it’s cold and dark outside. But here are some tips to help you make the most of your winter workouts:Sign up for a weekly class. Wasting money can be a good motivator to workout. I prepay for one class a week so no matter how I feel, I don’t miss that class.Not a fan of running outside in the winter and can’t get to a treadmill? Doing …