PROVIDENCE — As shopping center owners see weakness among their retail tenants, many are turning to fitness gyms to beef up their portfolios, experts on the local commercial real estate market say.As big-box stores close, many of them are being replaced by gyms, according to two Boston-based spokesmen for CoStar Group, a commercial real estate data firm headquartered in Washington, D.C.In fact, CoStar market analyst Peter Conway said, gyms lead all other new tenants in [...]