function mode( mu, sigma ) {
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return NaN;
	}
	return exp( mu - ( sigma*sigma ) );
}
Returns the mode for a lognormal distribution with location `mu` and scale `sigma`.

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

@example
var y = mode( 0.0, 1.0 );
// returns ~0.368

@example
var y = mode( 5.0, 2.0 );
// returns ~2.718

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

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

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