add method argument in posthoc_wilcoxon

[YongcaiHuang]

add method argument in posthoc_wilcoxon. the current function has no method argument to control how to calculate the p-value. Although Scipy default is auto, there are warnings like:
UserWarning: Exact p-value calculation does not work if there are zeros. Switching to normal approximation.
when set method as "approx", no warning shows and p values are slightly changed (may be more accurate)

method{“auto”, “exact”, “approx”}, optional
Method to calculate the p-value, see Notes. Default is “auto”.