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

@param {NonNegativeNumber} sigma - scale parameter
@returns {NonNegativeNumber} skewness

@example
var v = skewness( 9.0 );
// returns ~0.631

@example
var v = skewness( 2.0 );
// returns ~0.631

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

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