Simplify model comparison APIs

[sethaxen]

This PR simplifies the model comparison APIs. While the functionality being removed here may still be useful, removing these now allows us to reduce the support burden for the package, and we can add these back slowly if users request them:

• Removes model_weights from API (weights can still be computed and accessed using compare)
• Renames weights_method keyword for compare to simply method
• Removes information_criterion (TODO: document relationship between ELPD and information criteria)
• Removes AbstractELPDResult from the API
• Removes elpd_estimates (in favor of user accessing the estimates and pointwise fields of PSISLOOResult directly)

[sethaxen]

Going to hold off on merging this just yet since I'll probably refine the AbstractELPDResult interface in parallel to adding the missing LOO functionality.