Make OLS regression independent of time series frequency.

[abshinn]

Right now, rdtools.degradation_with_ols assumes the input time series is monthly. This function should be generalized to handle other time series frequencies.

[mdeceglie]

I have addressed this for degradation_with_ols in the issue 4 branch. I suggest continued development on that branch to generalize the other rd methods before merging. Once merged into master, we can close this issue.

[mdeceglie]

Closed with merge from March 2.