It calculates the drift value with given upper bound z-values, times of test, and power. If the times of test is not given, even-interval is assumed. mvtnorm::pmvt
(with noncentrality) is better than pmvnorm in calculating power and sample size. But, Lan-DeMets used multi-variate normal rather than multi-variate noncentral t distributionh. This function followed Lan-DeMets for the consistency with previous results.