Incremental PCA implementation

[MaxHalford]

Incremental PCA is already implemented in scikit-learn but seems like it is straightforward to implement. See this paper too.

[lbowenwest]

I'd like to work on this if that's alright

[MaxHalford]

You're on! Feel free to ask questions :). One thing to note is that this new PCA class will be part of a new module called decomposition.

I'm just about to review your PR for AdaBound :). We're also going to be adding a nice of contributors to the README.md to give your help some exposure.

[lbowenwest]

Cheers, yeah will do.

[MaxHalford]

Any progress @lbowenwest? If not I'll do it.

[MaxHalford]

This paper looks quite promising.

[MaxHalford]

Added to the icebox.

[microprediction]

Did this make it in?

[gbolmier]

No, there is some ongoing (or stale?) work in #1509 that apparently requires reviewing time.