Add previews and diagrams rules for pds4 uranus occ

[juzen2003]

Update:

• For PDS4 uranus occ:
  • Add default_viewables rules and corresponding test test_viewset
  • Add associations_to_previews and associations_to_diagrams rules and corresponding test test_associated_abspaths with previews and diagrams categories.
  • Include previews and diagrams files in the opus_products rules
    • Note: for previews and diagrams files to show up in the opus_products output, they must exist in the filesystem.
• For PDS4 Cassini: ignore tests, will update them once we know the final directory structures.