Function builtin_ppois 

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

Poisson cumulative distribution function.

ppois(q, lambda) = sum_{k=0}^{floor(q)} dpois(k, lambda) Uses the regularized gamma: ppois(k, lambda) = 1 - P(k+1, lambda)

@param q quantile vector @param lambda mean rate parameter @return numeric vector of probabilities