function standardDeviation(x) {
    if (x.length === 1) {
        return 0;
    }
    const v = variance(x);
    return Math.sqrt(v);
}
The [standard deviation](http://en.wikipedia.org/wiki/Standard_deviation)
is the square root of the variance. This is also known as the population
standard deviation. It's useful for measuring the amount
of variation or dispersion in a set of values.

Standard deviation is only appropriate for full-population knowledge: for
samples of a population, {@link sampleStandardDeviation} is
more appropriate.

@param {Array<number>} x input
@returns {number} standard deviation
@example
variance([2, 4, 4, 4, 5, 5, 7, 9]); // => 4
standardDeviation([2, 4, 4, 4, 5, 5, 7, 9]); // => 2