Update explorer to use upcoming OEPS data release

[mradamcox]

At the end of the summer, we will be making a new release of the OEPS data warehouse. This will include the addition of historical datasets, as well as a significant restructuring of the files themselves. The exact details of these changes are not yet clear, so this ticket is mostly a placeholder to point out that some pretty significant updates will need to be made here. Once the release is made (or at least once the OEPS main branch is much closer to what it will look like after the release) we can add some more details here.