function sampleStandardDeviation(x) {
    const sampleVarianceX = sampleVariance(x);
    return Math.sqrt(sampleVarianceX);
}
The [sample standard deviation](http://en.wikipedia.org/wiki/Standard_deviation#Sample_standard_deviation)
is the square root of the sample variance.

@param {Array<number>} x input array
@returns {number} sample standard deviation
@example
sampleStandardDeviation([2, 4, 4, 4, 5, 5, 7, 9]).toFixed(2);
// => '2.14'