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 "statistic" to clarify between
  observation w/ confidence interval and actually test statistic.
- adapt pretty print
- 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
- some reordering of functions
WIP Cleanup t-tests

- unify t-test code
- add some more information to t-test struct
- adapt t-test to pretty print
- add docu
- rename t-test names