function stdev( alpha, beta ) {
	if ( alpha <= 0.0 || beta <= 0.0 ) {
		return NaN;
	}
	return sqrt( alpha ) / beta;
}
Returns the standard deviation of a gamma distribution.

@param {PositiveNumber} alpha - shape parameter
@param {PositiveNumber} beta - rate parameter
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 1.0, 1.0 );
// returns 1.0

@example
var v = stdev( 4.0, 12.0 );
// returns ~0.167

@example
var v = stdev( 8.0, 2.0 );
// returns ~1.414

@example
var v = stdev( 1.0, -0.1 );
// returns NaN

@example
var v = stdev( -0.1, 1.0 );
// returns NaN

@example
var v = stdev( 2.0, NaN );
// returns NaN

@example
var v = stdev( NaN, 2.0 );
// returns NaN