When New Hampshire-born Will Sheff, frontman of veteran folk rockers Okkervil River, returns to Boston to play The Sinclair on Saturday, he’ll once again be performing in a mid-size venue to fans, relatives and old friends. This time, though, he’s returning to New England in support of an album inspired by his hometown, Meriden, New Hampshire, and his youth. “The Silver Gymnasium,” a nostalgic autobiography of a ten year-old Sheff in 1986, continues Okkervil River’s legacy as introspective, niche, career musicians. If Okkervil River don’t ever break mainstream — and if Sheff has a say, they may not want to — this album does put the frontman’s small town on the map. More