function kurtosis( mu, sigma ) {
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return NaN;
	}
	return 0.0;
}
Returns the excess kurtosis for a normal distribution with mean `mu` and standard deviation `sigma`.

@param {number} mu - mean
@param {PositiveNumber} sigma - standard deviation
@returns {number} excess kurtosis

@example
var y = kurtosis( 0.0, 1.0 );
// returns 0.0

@example
var y = kurtosis( 5.0, 3.0 );
// returns 0.0

@example
var y = kurtosis( NaN, 1.0 );
// returns NaN

@example
var y = kurtosis( 0.0, NaN );
// returns NaN

@example
var y = kurtosis( 0.0, 0.0 );
// returns NaN