function stdev( mu, sigma ) {
	var s2;
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return NaN;
	}
	s2 = sigma*sigma;
	return sqrt( ( exp( s2 ) - 1.0 ) * exp( ( 2.0*mu ) + s2 ) );
}
Returns the standard deviation for a lognormal distribution with location `mu` and scale `sigma`.

@param {number} mu - location parameter
@param {PositiveNumber} sigma - scale parameter
@returns {NonNegativeNumber} standard deviation

@example
var y = stdev( 0.0, 1.0 );
// returns ~2.161

@example
var y = stdev( 5.0, 2.0 );
// returns ~8028.534

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

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

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