TInv  - Script and chart function
                TInv() returns the t value of the student's t distribution as a function of the probability and the degrees of freedom.
Syntax:
TINV(prob, degrees_freedom)
Return data type: number
Arguments:
| Argument | Description | 
|---|---|
| prob | A two-tailed probability associated with the t-distribution. It must be a number between 0 and 1. | 
| degrees_freedom | An integer stating the number of degrees of freedom. | 
Limitations:
All arguments must be numeric, else NULL will be returned.
This function is related to the TDist 
 function in the following way: 
If prob = TDIST(value, df ,2), 
 then TINV(prob, df) = value.