feat: add stats/base/dists/bradford/mean

[anandkaranubc]

Progresses #168

Description

What is the purpose of this pull request?

This pull request:

• Adds the JS implementation of the expected value of a Bradford distribution.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves RFC: add the Bradford distribution #168

Questions

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No.

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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/bradford/mean	$\color{green}117/117$ $\color{green}+100.00\%$	$\color{green}6/6$ $\color{green}+100.00\%$	$\color{green}1/1$ $\color{green}+100.00\%$	$\color{green}117/117$ $\color{green}+100.00\%$

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

[anandkaranubc]

The tolerance had to be slightly increased due to differences in the log( 1 + c ) calculations when c was close to 0 while testing it against SciPy-generated test fixtures.

[anandkaranubc]

I feel like using log1p would be more appropriate here, but its use causes a greater divergence between our values and SciPy's compared to using ln. I tested it with log1p, and for all test cases to pass, we would need to increase the tolerance to approximately 60.0 * EPS.

[anandkaranubc]

This is more noticeable when c is close to 0.