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

[Jaysukh-409]

Description

What is the purpose of this pull request?

This pull request:

• Adds the mean for the Planck Discrete Random Variable.

Related Issues

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This pull request:

Questions

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No.

Other

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No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

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@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/planck/mean	$\color{green}108/108$ $\color{green}+100.00\%$	$\color{green}6/6$ $\color{green}+100.00\%$	$\color{green}1/1$ $\color{green}+100.00\%$	$\color{green}108/108$ $\color{green}+100.00\%$

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

[kgryte]

Most of the same requested changes in #3896 are applicable here.

[Jaysukh-409]

If generating rv doesn't need changes then this PR is ready to merge.

[kgryte]

/stdlib merge

[kgryte]

Left another round of review comments.

[kgryte]

/stdlib merge

[kgryte]

This now LGTM. Thanks for working on this @Jaysukh-409.