feat: add stats/incr/nanmvariance

[gkbishnoi07]

Resolves #5614

Description

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This pull request:

• add stats/incr/nanmvariance

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• resolves no related issues.

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/incr/nanmvariance	$\color{green}156/156$ $\color{green}+100.00\%$	$\color{green}11/11$ $\color{green}+100.00\%$	$\color{green}2/2$ $\color{green}+100.00\%$	$\color{green}156/156$ $\color{green}+100.00\%$

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

[gkbishnoi07]

@Planeshifter please review this PR

[kgryte]

You shouldn't be inlining the entire implementation of incrmvariance. Your implementation should be a wrapper around incrmvariance which provides specialized NaN handling.

[gkbishnoi07]

Got it working on it

[gkbishnoi07]

@kgryte, could you please review this? I've updated the implementation according to your feedback. Let me know if this looks correct or if there are any further changes needed. Thanks!