WIP Approach to clarify presentation of test functions

Currently, some of the basic concepts are not presented in the best
way; it is a bit cumbersome to distinguish between the observed
value to which the null hypothesis relates, the test statistic,
and the confidence interval. The goal of this commit is to make clear
how these quantities belong together and to ensure a consistent
implementation over all tests. Therefore, some suggestions:

- added and printed out meta function "population_param_of_interest"
  to clarify between observation w/ confidence interval and actually
  test statistic.
- add show_params to describe meta data of a test (e.g., contingence
  table) and unify pretty print
- group point estimate and confidence interval to test statistic
- unify t-test code
- adapt test to pretty print
WIP Separate Wilcoxon and Mann-Whitney test

These two tests are implemented in the same file, since they use the
same functions, but are actually different. However, to make
the presentation more clear, they should be in separated files.

- add common file to extract helper functions
- clean up Wilcoxon and Mann-Whitney
- adapt function to pretty print