function stdev( k, lambda ) {
	var g1k;
	if (
		isnan( k ) ||
		isnan( lambda ) ||
		k <= 0.0 ||
		lambda <= 0.0
	) {
		return NaN;
	}
	g1k = gamma( 1.0 + ( 1.0/k ) );
	return lambda * sqrt( gamma( 1.0 + ( 2.0/k ) ) - ( g1k*g1k ) );
}
Returns the standard deviation of a Weibull distribution.

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

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

@example
var v = stdev( 4.0, 12.0 );
// returns ~3.051

@example
var v = stdev( 8.0, 2.0 );
// returns ~0.279

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

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

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

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