code: @Reference(authors = "D. J. Best, D. E. Roberts", //
        title = "Algorithm AS 91: The percentage points of the χ² distribution", //
        booktitle = "Journal of the Royal Statistical Society. Series C (Applied Statistics)", //
        url = "https://doi.org/10.2307/2347113", //
        bibkey = "doi:10.2307/2347113")
    public static double quantile(double x, double dof) {
      return GammaDistribution.quantile(x, .5 * dof, .5);
    }
docs: Return the quantile function for this distribution
  <p>
  Reference:
  <p>
  D. J. Best, D. E. Roberts<br>
  Algorithm AS 91: The percentage points of the χ² distribution<br>
  Journal of the Royal Statistical Society. Series C (Applied Statistics)
  
  @param x Quantile
  @param dof Degrees of freedom
  @return quantile position