Update mannwhitneyu such that it does not throw an error when the two arrays are identical and contains a single value

[Hu-JIN]

This rarely happens. But in some cases, say we have x = [0, 0, 0], and y = [0, 0, 0], stats.mannwhitneyu(x, y) will throw an error. Let's write a wrapper to take care of that. This is similar to what we've already done in differential_tail_test().