Series vs. TimeSeries methods

[jseabold]

Just looking at a few of these after PR #1991. Should autocorr and autocov be methods of TimeSeries rather than Series since not all Series are going to be TimeSeries? Maybe at least head in this direction?

[wesm]

There might be cases where the data is a "time series" but the index is not necessarily of the right type (suppose it's integer time stamps of some kind). I'm honestly less and less thrilled about the existence of the TimeSeries data type at all

[jreback]

#3482 got rid of the distinct type