function median( alpha, beta ) {
	if ( alpha <= 0.0 || beta <= 0.0 ) {
		return NaN;
	}
	return betaincinv( 0.5, alpha, beta );
}
Returns the median of a beta distribution.

@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {PositiveNumber} median

@example
var v = median( 1.0, 1.0 );
// returns 0.5

@example
var v = median( 4.0, 12.0 );
// returns ~0.239

@example
var v = median( 8.0, 2.0 );
// returns ~0.820

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

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

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

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