GammaDensity  - Script and chart function
                    GammaDensity() returns the probability of the Gamma distribution.
Syntax:
GammaDensity(value, k, θ)
Return data type: number
| Argument | Description | 
|---|---|
| value | The value at which you want to evaluate the distribution. The value must not be negative. | 
| 
                                     k  | 
                                A positive number defining the shape parameter. | 
| θ | A positive number defining the scale parameter. |