The story lines in “Treme” begin three months after Katrina, and they follow a diverse group of characters as they rebuild their lives in a city torn apart, a city in which tens of thousands of houses are abandoned, in which only 50 percent of the population remains, in which neighborhoods are still without power. The main characters in “Treme” aren’t the overburdened cops, spiraling addicts, ruthless dealers, struggling dockworkers, corrupt politicians or compromised journalists of “The Wire.” In their place, for the most part, are musicians.