Function builtin_pf 

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

F cumulative distribution function.

Uses the regularized incomplete beta function: pf(x, d1, d2) = I_{d1x/(d1x+d2)}(d1/2, d2/2)

@param q quantile vector @param df1 numerator degrees of freedom @param df2 denominator degrees of freedom @return numeric vector of probabilities