Add Additional Calculations & Rolling Periods

[cgdeboer]

@twiecki and friends, I'm working on the project that requires many of the same calculations that are performed in this repo, but also several others. I am starting to work on a PR to add a variety of other statistical measures as well as a framework for getting rolling metrics on those things.

Here are a few examples of additional metrics: up-capture, down-capture, tracking-error.

Would like to add some ability to create rolling metrics. eg: Rolling 60-day Max Drawdown, Rolling 12 month Tracking Error

Before I start this work in earnest, I want to ensure that you all are open to expanding the calculation and capability set. Also, I can include a correct calculation of information ratio.

[twiecki]

@cgdeboer That would be much appreciated and we'd be more than happy to work with you on this.

[cgdeboer]

Great ! I'll post an initial PR tomorrow or early next week as a "Work In Progress" so I can get some initial thoughts from you.