Nathaniel Kramer, the Allen & Co. banking heir, will host a private screening tonight at Soho House of his documentary, “A Long Haul,” the touching story of a commercial fisherman on the East End struggling to make ends meet in a bad economy and an overfished ocean. By his side is his loyal mate, a 13-year-old boy from a wealthy family working as a apprentice all summer long. Kramer — who wrote, directed, produced and edited the film — spent many years in shallower waters as a fashion photographer.