unit tests added

1. test on the effect sizes&pval calculated
2. test on the plots

Author: LI-Yixuan

dabest/_classes.py

@@ -765,7 +765,7 @@ def __init__(self, control, test, effect_size,
             err2 = "is not one of {}".format(kosher_es)
             raise ValueError(" ".join([err1, err2]))
 
-        if effect_size == "cliffs_delta" and paired:
+        if effect_size == "cliffs_delta" and is_paired:
             err1 = "`paired` is not None; therefore Cliff's delta is not defined."
             raise ValueError(err1)