function stdev( sigma ) {
	if ( isnan( sigma ) || sigma < 0 ) {
		return NaN;
	}
	return SQRT4MPI * sigma / SQRT2;
}
Returns the standard deviation of a Rayleigh distribution.

@param {NonNegativeNumber} sigma - scale parameter
@returns {NonNegativeNumber} standard deviation

@example
var v = stdev( 9.0 );
// returns ~5.896

@example
var v = stdev( 2.0 );
// returns ~1.31

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

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