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

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

@example
var v = mode( 9.0 );
// returns 9.0

@example
var v = mode( 2.0 );
// returns 2.0

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

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