function kurtosis( mu, sigma ) {
	return addon( mu, sigma );
}
Returns the excess kurtosis for a lognormal distribution with location `mu` and scale `sigma`.

@private
@param {number} mu - location parameter
@param {PositiveNumber} sigma - scale parameter
@returns {number} excess kurtosis

@example
var y = kurtosis( 0.0, 1.0 );
// returns ~110.936

@example
var y = kurtosis( 5.0, 2.0 );
// returns ~9220556.977

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

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

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