function kurtosis( lambda ) {
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	return 1.0 / lambda;
}
Returns the excess kurtosis of a Poisson distribution.

@param {PositiveNumber} lambda - mean parameter
@returns {PositiveNumber} excess kurtosis

@example
var v = kurtosis( 9.0 );
// returns ~0.111

@example
var v = kurtosis( 1.0 );
// returns 1.0

@example
var v = kurtosis( -0.2 );
// returns NaN

@example
var v = kurtosis( NaN );
// returns NaN