function mean( sigma ) {
	if ( isnan( sigma ) || sigma < 0.0 ) {
		return NaN;
	}
	return sigma * SQRT_HALF_PI;
}
Returns the expected value of a Rayleigh distribution.

@param {NonNegativeNumber} sigma - scale parameter
@returns {NonNegativeNumber} expected value

@example
var v = mean( 9.0 );
// returns ~11.28

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

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

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