Add installing package and pointing the database to github about posteriordb HOT 10 CLOSED

I guess this should be implemented by downloading models, datasets etc to a cache or data directory. Probably only the models the user uses should be downloaded.

I found a python package for accessing platform specific data/cache/config directories, https://github.com/ActiveState/appdirs

For example for the user trentm the cache dir on MacOS would be /Users/trentm/Library/Caches/posteriordb, on Windows C:\Users\trentm\AppData\Local\author_name\posteriordb\\Cache (author_name could also be posteriordb) and on Linux /home/trentm/.cache/posteriordb

I'm not sure if it would be better to use the cache dir or the data dir.

The main technical challenge is probably determining when a downloaded file is stale and needs to be downloaded again. One approach would be having a file in this repo that contains hashes of each file in the posterior database. Then with the hash we could be determine if a file needs to be re-downloaded. There are other approaches too, such as just redownloading everything once a new version of posteriordb is released.

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Another possibility would be to include the whole PDB in the R and python packages (or as a separate posteriordbdata package). With this approach we wouldn't need to worry about platform-specific directories, but using this would mean that that the package needs to be updated to access new models etc and also that the whole database is always downloaded even if the user wants to use only a few posteriors

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I feel a persistent cache is the way to go, having to redownload the models and datasets just because R session was closed doesn't feel right to me. The implementation effort of the persistent cache also doesn't seem large to me.

So with the git sha approach we redownload everything when the PDB gets new commits? I agree that for the first version this is good enough, it can be improved later.

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A few questions:

1. If the user calls pos <- posterior_names(my_pdb) is it OK that all the posterior info files in posteriors/ get downloaded? And same with model_names and dataset_names

2. Is it best to not yet download the dataset file and model code when the user calls po <- posterior("8_schools-8_schools_centered", my_pdb) and instead download them when the user calls dataset(po) or stan_code(po)? And the same applies for downloading the model info and dataset info files.

   The posterior info however would be downloaded at this point.

   A downside to this approach is that it can be hard to validate that the posterior has valid dataset and model code.

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This has now been solved. It just needs to be updated in the vignette and documentation.

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Good job!

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To eeros comment above. I have now added a cache_path to the pdb object so it is can be stored wherever. If we want we can point the local db path to be the cache, then nothing is cached.

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