feat: add stats/nanmeanwd

[Orthodox-64]

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Resolves None.

Description

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This pull request:

• feat: add stats/nanmeanwd

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/nanmeanwd	$\color{green}164/164$ $\color{green}+0.00%$	$\color{green}2/2$ $\color{green}+0.00%$	$\color{green}0/0$ $\color{green}+0.00%$	$\color{green}164/164$ $\color{green}+0.00%$

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

[kgryte]

You should add the references for Welford's algorithm (see stats/base/ndarray/nanmeanwd).

[kgryte]

Left initial comments.

[Orthodox-64]

@kgryte Done with the changes. Could you please review the dmeanstdev and nanmax-by pr as well?