function quantile( p, k ) {
	return gammaQuantile( p, k/2.0, 0.5 );
}
Evaluates the quantile function for a chi-squared distribution with degrees of freedom `k` at a probability `p`.

@param {Probability} p - input value
@param {NonNegativeNumber} k - degrees of freedom
@returns {number} evaluated quantile function

@example
var y = quantile( 0.8, 1.0 );
// returns ~1.642

@example
var y = quantile( 0.5, 4.0 );
// returns ~3.357

@example
var y = quantile( 0.8, 0.1 );
// returns ~0.014

@example
var y = quantile( -0.2, 0.5 );
// returns NaN

@example
var y = quantile( 1.1, 0.5 );
// returns NaN

@example
var y = quantile( NaN, 1.0 );
// returns NaN

@example
var y = quantile( 0.0, NaN );
// returns NaN

@example
// Negative degrees of freedom:
var y = quantile( 0.5, -1.0 );
// returns NaN