function skewness( k, lambda ) {
	var sigma2;
	var sigma;
	var out;
	var mu;
	if (
		isnan( k ) ||
		isnan( lambda ) ||
		k <= 0.0 ||
		lambda <= 0.0
	) {
		return NaN;
	}
	mu = mean( k, lambda );
	sigma2 = variance( k, lambda );
	sigma = sqrt( sigma2 );
	out = gamma( 1.0 + ( 3.0/k ) ) * pow( lambda, 3.0 );
	out -= ( 3.0*mu*sigma2 ) + pow( mu, 3.0 );
	out /= pow( sigma, 3.0 );
	return out;
}
Returns the skewness of a Weibull distribution.

@param {PositiveNumber} k - shape parameter
@param {PositiveNumber} lambda - scale parameter
@returns {number} skewness

@example
var v = skewness( 1.0, 1.0 );
// returns 2.0

@example
var v = skewness( 4.0, 12.0 );
// returns ~-0.087

@example
var v = skewness( 8.0, 2.0 );
// returns ~-0.534

@example
var v = skewness( 1.0, -0.1 );
// returns NaN

@example
var v = skewness( -0.1, 1.0 );
// returns NaN

@example
var v = skewness( 2.0, NaN );
// returns NaN

@example
var v = skewness( NaN, 2.0 );
// returns NaN