Implementing Autocorrelation Function

[timothydmorton]

Autocorrelation implementation, courtesy of Stephanie Douglas.

[timothydmorton]

Looking into this now; will re-open pull request when ready.

[timothydmorton]

Implemented basic autocorrelation function here. Catch is that it requires evenly-sampled data. Also desirable would be to implement the Scargle (1989) unevenly-sampled ACF algorithm.

[jakevdp]

regarding Scargle's method, take a look at this: https://github.com/astroML/astroML/blob/master/astroML/time_series/ACF.py

[timothydmorton]

In the ACF_scargle_test notebook (https://raw.githubusercontent.com/timothydmorton/periodogram/acf/examples/ACF_scargle_test.ipynb), I put in a simple test demo; the power seems to be inverted, which seems a bit funky.

[timothydmorton]

Trying to test the other ACF algorithms with the LINEAR data here: https://raw.githubusercontent.com/timothydmorton/periodogram/acf/examples/ACFExample.ipynb... I think I have to understand how these work a bit better before being able to give good examples.