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

@private
@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