feat: add Wald distribution mgf package

[manit2004]

Resolves #209

Description

What is the purpose of this pull request?

This pull request:

• Complete package for Wald mode has been implemented.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• None.

Questions

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No.

Other

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SciPy and Julia currently doesn't have mgf for Wald or Inverse Gaussian, so I had to generate fixtures from the formula itself. Scipy Wald, Scipy Inverse Gaussian, Julia Inverse Gaussian

Checklist

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• Read, understood, and followed the contributing guidelines.

AI Assistance

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• No

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• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/wald/mgf	$\color{green}314/314$ $\color{green}+0.00\%$	$\color{green}23/23$ $\color{green}+0.00\%$	$\color{green}4/4$ $\color{green}+0.00\%$	$\color{green}314/314$ $\color{green}+0.00\%$

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