FR: X2contingency

[omaus]

Please reference the issue(s) this PR is related to

Closes #180

Please list the changes introduced in this PR

• Introduces the X² test for 2×2 contingency tables aka Cochran-Mantel-Haenszel test

Description

An implementation of the X² test for 2×2 contingency tables is added alongside with corresponding unit tests.

• [X] The project builds without problems on your machine
• [X] Added unit tests regarding the added features