Expanding window statistics

[changhiskhan]

analogs to all the 'rolling_' functions

will be nearly identical setup with some Cython code to do _expanding_moment

[qwhelan]

Hi,

Is there a particular reason why this issue was closed?

I've implemented most of the expanding_ functions by simply calling the analogous rolling_ function with window=len(arg) and min_periods defaulting to the minimum necessary to compute the statistic (1 for most, 4 for kurtosis, etc.). The only potentially surprising output I've run into is that NaNs are ignored after the first min_periods while the NumPy equivalents (like cummax) occasionally preserve them.

I'll submit a pull request if this seems like a reasonable solution.

[wesm]

I merged in the PR. Thanks