Nashville is back in the Big Apple, part of a big bet on country music’s increasingly mainstream appeal by Cumulus Media. For the first time in at least a decade, the New York City market has an all country music station. Cumulus on Monday turned its WRXP station, which lately was airing the rock and pop music programming of another Cumulus station, into an all country station that it plans to call WNSH. The switch is part of an effort by Cumulus, the second-biggest radio broadcaster by stations after Clear Channel Communications Inc., to begin converting all of its now 83 country stations across the nation to the newly created NASH brand starting on Tuesday.