function kurtosis( sigma ) {
	if ( isnan( sigma ) || sigma < 0.0 ) {
		return NaN;
	}
	return KURTOSIS;
}
Returns the excess kurtosis of a Rayleigh distribution.

@param {NonNegativeNumber} sigma - scale parameter
@returns {NonNegativeNumber} excess kurtosis

@example
var v = kurtosis( 9.0 );
// returns ~0.245

@example
var v = kurtosis( 5.0 );
// returns ~0.245

@example
var v = kurtosis( 3.0 );
// returns ~0.245

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

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