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

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

@example
var m = mean( 9.0 );
// returns ~11.2798

@example
var m = mean( 2.0 );
// returns ~2.507

@example
var m = mean( -0.2 );
// returns NaN

@example
var m = mean( NaN );
// returns NaN