Calculates normal confidence intervals for a given alternative at a given significance level.
zconfint_generic(mean, std_mean, alpha, alternative)(vector): Lower and upper (1 - alpha) * 100% confidence limits.
(float): Estimated normal mean.
(float): Estimated standard error of the mean.
(float): Significance level in [0,1]
(string): Alternative hypothesis, either 'two-sided', 'larger' or 'smaller'.