GammaInv  - script and chart function
                    GammaInv() returns the inverse of the accumulated probability of the Gamma distribution.
Syntax:
GammaInv(prob, k, θ)
Return data type: number
| Argument | Description | 
|---|---|
| prob | A probability associated with the Gamma-probability distribution. It must be a number between 0 and 1. | 
| 
                                     k  | 
                                A positive number defining the shape parameter. | 
| θ | A positive number defining the scale parameter. | 
This function is related 
			to the GammaDist function in the following way: 
If prob = GammaDist(value, k, θ), then GammaInv(prob, k, θ) = value