x1, x2 : array_like, 1D or 2D
two independent samples, see notes for 2D case
value : float
In the one sample case, value is the mean of x1 under the Null
hypothesis.
In the two sample case, value is the difference between mean of x1 and
mean of x2 under the Null hypothesis. The test statistic is
x1_mean  x2_mean  value.
usevar : string, ‘pooled’
Currently, only ‘pooled’ is implemented.
If pooled, then the standard deviation of the samples is assumed to be
the same. see CompareMeans.ztest_ind for different options.
ddof : int
Degrees of freedom use in the calculation of the variance of the mean
estimate. In the case of comparing means this is one, however it can
be adjusted for testing other statistics (proportion, correlation)
