function stdev( alpha, beta ) {
	if ( alpha <= 2.0 || beta <= 0.0 ) {
		return NaN;
	}
	return beta / ( ( alpha-1.0 ) * sqrt( alpha-2.0 ) );
}
Returns the standard deviation of an inverse gamma distribution.

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

@example
var v = stdev( 3.0, 5.0 );
// returns ~2.5

@example
var v = stdev( 4.0, 12.0 );
// returns ~2.828

@example
var v = stdev( 8.0, 2.0 );
// returns ~0.117

@example
var v = stdev( 3.0, -0.1 );
// returns NaN

@example
var v = stdev( 1.5, 1.0 );
// returns NaN

@example
var v = stdev( 3.0, NaN );
// returns NaN

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