James Ponsoldt is expecting this question. Why did the guy who made films like “Smashed,” “The Spectacular Now,” and “The End of the Tour” — dark, knowing dramas about messed-up adults, typically with substance abuse problems and a host of neuroses — turn his attention to “Summering,” a film about four tween girls in the waning days of their favorite season? He’s got the answer in hand: He’s a parent of three kids, his wife Megan works in the public-school system, and this is the stuff he wants to share with his family. But the real answer?