ExtremeValue(Observations, Recurrence_Interval)
Returns estimated observed value for given recurrence interval using the Extreme Value (Gumbel) distribution
Input:       Observations = List of observations such as annual precipitation (numbers)                  
Recurrence_Interval = Recurrence interval for which to estimate the observed value Output:     
Predicted observation for given recurrence interval calculated using the |
Notes: See Extreme Value Type I Distribution for additional information.