Improve PDS3 integration

[mdrum]

The PDS3 search results on each page are an extremely basic name-based search, which has already proven to have many false positives or missing results. Since we're not going to successfully migrate all datasets to PDS4 for quite a while, a deeper integration with PDS3 might be needed.

Several approaches we could take:

• Manually assign each context object its PDS3 LID, and then replace the current results with ones from that LID
• Further integrate dataset search results and display pages with PDS3, and make the model version opaque to the user
• Improve the registry such that PDS3 datasets are directly returned for queries involving PDS4 LIDs

[mdrum]

This has now been implemented in the latest pull request #9