POISSON function

 Returns the Poisson distribution


 * $$P(k \text{ events in interval}) = e^{-\lambda}\frac{\lambda^k}{k!}$$


 * POISSON(x,mean,cumulative)
 * X - the number of events.
 * Mean - the expected numeric value.
 * Cumulative - a logical value that determines the form of the probability distribution returned.
 * If cumulative is TRUE, POISSON returns the cumulative Poisson probability that the number of random events occurring will be between zero and x inclusive.
 * If cumulative is FALSE, it returns the Poisson probability mass function that the number of events occurring will be exactly x.
 * If x is not an integer, it is truncated.
 * If x or mean is nonnumeric, POISSON returns the #VALUE! error value.
 * If x < 0, POISSON returns the #NUM! error value.
 * If mean < 0, POISSON returns the #NUM! error value.