function median( k ) {
	return quantile( 0.5, k );
}
Returns the median of a chi-squared distribution.

@param {NonNegativeNumber} k - degrees of freedom
@returns {NonNegativeNumber} median

@example
var v = median( 9.0 );
// returns ~8.343

@example
var v = median( 2.0 );
// returns ~1.386

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

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