Renaming XXXESSMethod to XXXAutocovMethod

[sethaxen]

All of our AbstractESSMethods really just define different approaches for estimating the mean autocovariance of demeaned samples, whereas optional arguments, keyword arguments, and other methods (see #53) are used to define different ESS variants (ESS of different estimands, ESS with preprocessing, splitting vs unsplit, maxlag, etc).

For clarity, I suggest we rename AbstractESSMethod to AbstractAutocovMethod, following a similar pattern for the subtypes. We can rename the method kwarg passed to ess to autocov_method.

[sethaxen]

Fixed by #73