feat: add implementation of stats/base/dists/wald/skewness

[bhargava-d16]

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Progresses #209

Description

What is the purpose of this pull request?

This pull request:

• adds an implementation of the skewness for the Wald (Inverse Gaussian) distribution.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• RFC: add the Wald distribution #209

Questions

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No.

Other

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No.

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• Read, understood, and followed the contributing guidelines.

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/wald/skewness	$\color{green}185/185$ $\color{green}+0.00%$	$\color{green}10/10$ $\color{green}+0.00%$	$\color{green}2/2$ $\color{green}+0.00%$	$\color{green}185/185$ $\color{green}+0.00%$

The above coverage report was generated for the changes in this PR.

[Planeshifter]

Thanks!

[Planeshifter]

Looks like there are a couple of copy-paste issues on this line - the equation label says rayleigh_skewness and the alt text says "Rayleigh distribution" instead of Wald.

Suggested change

	<!-- <equation class="equation" label="eq:rayleigh_skewness" align="center" raw="\operatorname{skew}\left( X \right) = 3 \sqrt{\frac{\mu}{\lambda}}" alt="Skewness for a Rayleigh distribution."> -->
	<!-- <equation class="equation" label="eq:wald_skewness" align="center" raw="\operatorname{skew}\left( X \right) = 3 \sqrt{\frac{\mu}{\lambda}}" alt="Skewness for a Wald distribution."> -->

[Planeshifter]

Same kind of copy-paste issue here - the data-equation references the logistic distribution instead of Wald.

Suggested change

	<!-- <div class="equation" align="center" data-raw-text="\operatorname{skew}\left( X \right) = 3 \sqrt{\frac{\mu}{\lambda}}" data-equation="eq:logistic_skewness">
	<!-- <div class="equation" align="center" data-raw-text="\operatorname{skew}\left( X \right) = 3 \sqrt{\frac{\mu}{\lambda}}" data-equation="eq:wald_skewness">

[Planeshifter]

The printf label says "Mean" but this is computing skewness. The README's C example (line 215) has this correct already.

Suggested change

	printf( "µ: %.4f, λ: %.4f, Mean(X;µ,λ): %.4f\n", mu, lambda, y );
	printf( "µ: %.4f, λ: %.4f, Skewness(X;µ,λ): %.4f\n", mu, lambda, y );