Function builtin_qf 

fn builtin_qf(
    args: &[RValue],
    named: &[(String, RValue)],
) -> Result<RValue, RError>

F quantile function (inverse CDF).

Uses bisection on the CDF.

@param p probability vector @param df1 numerator degrees of freedom @param df2 denominator degrees of freedom @return numeric vector of quantiles