In the handbook of private equity investing techniques, few make more intuitive sense than the “roll-up.” Borrow some money, buy up small players in a fragmented industry, cut costs, find synergies, juice profits, increase multiples, pay down debt, and sell at a profit. Financial operators have done it well in waste management, funeral homes, and advertising. Now Bain Capital (yes, the company that clean-living Mitt Romney used to run) is trying to consolidate an unlikely industry: addiction treatment centers.