function coefficientOfVariation(x) {
    return sampleStandardDeviation(x) / mean(x);
}
The`coefficient of variation`_ is the ratio of the standard deviation to the mean.
.._`coefficient of variation`: https://en.wikipedia.org/wiki/Coefficient_of_variation


@param {Array} x input
@returns {number} coefficient of variation
@example
coefficientOfVariation([1, 2, 3, 4]).toFixed(3); // => 0.516
coefficientOfVariation([1, 2, 3, 4, 5]).toFixed(3); // => 0.527
coefficientOfVariation([-1, 0, 1, 2, 3, 4]).toFixed(3); // => 1.247