function median( sigma ) {
	return addon( sigma );
}
Returns the median of a Rayleigh distribution.

@private
@param {NonNegativeNumber} sigma - scale parameter
@returns {NonNegativeNumber} median

@example
var v = median( 9.0 );
// returns ~10.597

@example
var v = median( 2.0 );
// returns ~2.355

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

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