function mean( mu, sigma ) {
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return NaN;
	}
	return exp( mu + ( ( sigma*sigma ) / 2.0 ) );
}
Returns the expected value for a lognormal distribution with location `mu` and scale `sigma`.

@param {number} mu - location parameter
@param {PositiveNumber} sigma - scale parameter
@returns {NonNegativeNumber} expected value

@example
var y = mean( 0.0, 1.0 );
// returns ~1.649

@example
var y = mean( 5.0, 2.0 );
// returns ~1096.633

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

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

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