std
Computes the cumulative distribution function for a standard normal distribution using Taylor approximation.
The approximation is accurate to absolute error less than 8 * 10^(-16).
Reference: Evaluating the Normal Distribution by George Marsaglia.
http://www.jstatsoft.org/v11/a04/paper
Return
the P(Z<=z) for standard normal
Parameters
z
the z-ordinate to be evaluated