Multiple comparisons problem corrections

[xandaau]

In the Tester class when one use multiple experimantal groups or several metrics only Bonferroni correction is supported.

It will be useful to implement some more complex and popular classic corrections for MCP (Holm, Benjamini–Hochberg, etc). It should be noted that current structure of `Tester`` may not be so convenient to add these corrections, so it will need to change the main class code. These corrections should be discussed before the implementation as well as the problem of correct confidence intervals calculation.