Include test for normality violation in t_test() and unianova()

[LKobilke]

Since we provide feedback about the violation of normality in the regress() function, the idea came up that it might be beneficial to include this information in the t-test() and unianova() functions as well.

[MarHai]

Good idea. This would also allow us to streamline the code a bit -- and might thus be worth taking a look at before the christmas submit?