normsinvLibrary   "normsinv" 
 Description:  
Returns the inverse of the standard normal cumulative distribution. 
The distribution has a mean of zero and a standard deviation of one; i.e., 
normsinv seeks that value z such that a normal distribtuion of mean of zero 
and standard deviation one is equal to the input probability.
 Reference: 
github.com
 normsinv(y0) 
  Returns the inverse of the standard normal cumulative distribution. The distribution has a mean of zero and a standard deviation of one.
  Parameters:
     y0 : float, probability corresponding to the normal distribution.
  Returns: float, z-score
