feat: add C ndarray interface and refactor implementation for stats/base/dsnanmeanpn

[Neerajpathak07]

Resolves none.

Description

What is the purpose of this pull request?

This pull request:

• adds C ndarray interface for stats/base/dsnanmeanpn
• refactor JavaScript implementation
• refactor examples, tests, docs and benchmarks.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves none

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/dsnanmeanpn	$\color{green}399/399$ $\color{green}+100.00\%$	$\color{green}26/26$ $\color{green}+100.00\%$	$\color{green}4/4$ $\color{green}+100.00\%$	$\color{green}399/399$ $\color{green}+100.00\%$

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

[aayush0325]

thank you!

[kgryte]

LGTM