Using biased autocov for all lags

[sethaxen]

Currently we use the estimate of autocov with denominator n-1, which is unbiased for lag k = 0 and biased for k > 1:

MCMCDiagnosticTools.jl/src/ess.jl

Lines 10 to 13 in 57cacc1

	It is is based on the discussion by Vehtari et al. and uses the
	biased estimator of the autocovariance, as discussed by Geyer.
	In contrast to Geyer, the divisor `n - 1` is used in the estimation of
	the autocovariance to obtain the unbiased estimator of the variance for lag 0.

As noted, Geyer and Vehtari suggest using n as the denominator, not n-1. This is also used in Stan's, posterior's, and ArviZ's implementations of ESS. Unless we have a reference with simulations for our choice choice, I propose we instead use n-1 as suggested by these sources.

With #58 and #53, this is the last change needed to make our estimates identical to posterior's and ArviZ's within floating point precision.

[devmotion]

Not sure where the n - 1 originally comes from. Maybe since the FFT implementation does not require any rescaling (assuming they yield identical values, which I think they do)?

In any case, I don't think there's a convincing argument (yet) to deviate from the standard software in the area.