“A Brilliant Young Mind” is in many ways a British “Little Man Tate.” For those unfamiliar with “Tate,” it was Jodie Foster’s endearing, compassionate, witty 1991 film about a young boy whose genius sets him apart from others. He’s got a good heart, but he just doesn’t fit in. It’s much the same situation with “A Brilliant Young Mind,” a film about gifts and limitations. Nathan Ellis (played as a teen by Asa Butterfield of “Hugo” and “Ender’s Game,” and as a 9-year-old by Edward Baker-Close) has