[scikit-learn] partial_fit implementation for IsolationForest

[donkey-hotei at cryptolab.net]

nicolas,

> There is a good review on methods to do online random forests here:
> 
> https://arxiv.org/pdf/1302.4853.pdf
> 
> In fact, it turns out that the method of having a "window" of trees is
> not the best way to do. Usually the trees have to be grown in the same
> time data arrive, see
> 
> http://lrs.icg.tugraz.at/pubs/saffari_olcv_09.pdf
> 
> Adapting ensembles API to online learning seems hard work. But you can
> open a PR to discuss it.

Thanks a lot for the papers and info. I'll open PR at sometime and see 
what happens..

ty,
isaak