Co-living has made a small, quiet return to the Philadelphia market, but executives in the sector say it could be a decade before it catches fire in the same way it has overseas and in a handful of large, expensive metros like New York City and Washington, D.C. A pair of co-living spaces are in the works or already operating in the city, and a push to add more single-room occupancy units to Philly's housing stock has been a focus for several years. Yet the concept, which typically offers...