Backwards compatibility

Thank you all for the great package! It is really lovely to be able to include `pmlb` in a project and only fetch datasets as need (while avoiding having to personally host and provide access to others directly). Recently I ran into the situation that older versions of `pmlb` now return 404 errors, and when upgrading `pmlb` some datasets no longer exist or have been renamed. In general, not a huge deal for ad-hoc projects, but `pmlb` provides this great functionality for some longer term artifacts.

With that in mind, is there any interest in receiving a patch for (some amount) of backwards compatibility? It would be great if datasets that were removed because they are duplicates re-directed to their canonical names, and similarly that failed requests made a best-efforts attempt to provide an alternative request that succeeds.

I'd be happy to take a look at this, but wanted to gauge interest first (in the meantime I've found myself having to include old datasets directly in repos).