Use posterior::autocovariance() once on CRAN

[jgabry]

Remember to replace the short term fix for issue #151 (in PR #155) by using posterior::autocovariance() once the posterior package is on CRAN. This will avoid having to maintain the same code in both packages.

[avehtari]

posterior is on CRAN. What needs to be done to add the dependency correctly?

[avehtari]

@jgabry reminding about this issue you had opened

[jgabry]

Thanks. I will make a PR For this

[jgabry]

Thanks. I will make a PR For this

#215

[jgabry]

Reopening this. For the CRAN submission I'm about to do I'm going to add using posterior::autocovariance if the posterior package is available but we won't require it. In the future we can decide if we want to require posterior (i.e., move it from Suggests to Imports), which should depend on how much we plan to use posterior inside loo. If we end up requiring posterior we can delete the autocovariance function from loo.