Add Wilcoxon Distribution

[zieglerSe]

Description

Removed the obsolete Wilcoxon Distribution, as it is not used in the signed-rank tests.

[bvenn]

The signed rank test currently is only an approximation for large values (see #182 (comment))

There is a valid wilcoxon distribution that is quite expensive to calculate. Nevertheless it is required to calculate exact u test results, and should be implemented:

References:

• Rwilcox
• R vigenette
• analytica wilcox
• original publication