RFC: adaptive PCA

## Checklist

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## Description

> Description of the issue (or feature request).

This RFC proposes adding support for "adaptive PCA", as articulated [here](https://arxiv.org/pdf/1709.02373.pdf). Adding support for such an algorithm would allow, e.g., incremental computation or use within a streaming context.

Package: `@stdlib/ml/incr/pca`
Alias: `incrpca`

## Related Issues

> Does this issue (or feature request) have any related issues?

No.

## Questions

> Any questions for reviewers?

No.

## Other

> Any other information relevant to this issue (or feature request)? This may include screenshots, references, stack traces, sample output, and/or implementation notes.

No.

[code-of-conduct]: https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md

[faq]: https://github.com/stdlib-js/stdlib/blob/develop/FAQ.md