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

[anandkaranubc]

Progresses #168

Description

What is the purpose of this pull request?

This pull request:

• Adds the JS implementation of the cumulative density function (CDF) for a Bradford distribution, along with its factory implementation.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves RFC: add the Bradford distribution #168

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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/cdf	$\color{green}229/229$ $\color{green}+100.00\%$	$\color{green}23/23$ $\color{green}+100.00\%$	$\color{green}3/3$ $\color{green}+100.00\%$	$\color{green}229/229$ $\color{green}+100.00\%$

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

[anandkaranubc]

The use of log1p here SIGNIFICANTLY improves the tolerances (from a factor of ~5000 to ~5), unlike in the implementation of the mean, where it has the opposite effect.

I suspect it is because when the parameter values are too small, dividing two natural logarithms results in an extremely small value, leading to high numerical instability.

Just checked, SciPy uses log1p instead of ln in all their Bradford functions.