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

[github-actions]

🤖 This is an automated response from Repo Assist.

The "Wilcoxon distribution" referenced here is the exact null distribution of the Wilcoxon rank-sum statistic (used for the two-sample Mann-Whitney U test), distinct from the approximate normal distribution the signed-rank test currently uses.

What's needed:

• A DP-based CDF for the Wilcoxon rank-sum distribution: count the number of m-subsets of {1,...,m+n} that sum to ≤ w (the R wilcox.c algorithm). This is O(m·n·m·n) in the worst case but very fast for small samples.
• An exact variant of MannWhitneyUTest.computeUtest that uses the exact CDF for small samples (say m·n ≤ 400) and falls back to the normal approximation otherwise.
• PR FR: Mann-Whitney-U test #182 (omaus/FSharp.Stats) added a UTest.fs with a normal-approximation U-test but has had merge conflicts since 2022 — the implementation logic is sound and could be the starting point.

I'll look at implementing this in a future run, as I already have an open PR batch for this cycle.

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