Add new preset estimators, metrics, and cv iterators

Sklearn has a TON of estimators, metrics, and cv iterators that could trivially be added to the xcessiv.presets package. I'm a bit focused on other issues to bother adding them all.

Anyone who can help add to the list can easily do so.

Adding preset estimators/metrics/cvs is very easy. There's literally no need to understand how the rest of Xcessiv works, just take a look and copy the patterns in the xcessiv.presets package. Also, add corresponding relevant tests for your addition.

Please keep PR's limited to one feature addition only for easy debugging and reformatting if needed. Of course, you can submit as many PR's as you like :)