function variance( k, lambda ) {
	var mu;
	if (
		isnan( k ) ||
		isnan( lambda ) ||
		k <= 0.0 ||
		lambda <= 0.0
	) {
		return NaN;
	}
	mu = mean( k, lambda );
	return ( lambda*lambda * ( gamma( 1.0 + (2.0/k) ) ) ) - ( mu*mu );
}
Returns the variance of a Weibull distribution.

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

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

@example
var v = variance( 4.0, 12.0 );
// returns ~9.311

@example
var v = variance( 8.0, 2.0 );
// returns ~0.078

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

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

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

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