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

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

@example
var v = variance( 9.0 );
// returns ~34.765

@example
var v = variance( 2.0 );
// returns ~1.717

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

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