nums = array(((0.01, 0.01, 0.02, 0.04, 0.03), (0.0, 0.02, 0.02, 0.03, 0.02),
    (0.01, 0.02, 0.02, 0.03, 0.02), (0.01, 0.0, 0.01, 0.05, 0.03)))
How to efficiently calculate a running standard deviation?