When it comes to Elon Musk, it can be hard to separate the man from the myth. But in her new podcast, “The Evening Rocket,” Harvard historian and New Yorker writer Jill Lepore manages to see through Musk’s mystique, explain his worldview, and decipher his visions of the future by going back to the sci-fi stories he grew up on — stories, Lepore says, that Musk sometimes misread.