Make it convenient to select PDUs / CCVs / CCV IDs

Some pages will need to select a set of PDUs or even sets of CCV IDs.

One convenient default approach: select detector, get current PDU mapping, get most recent. What other options would we want to add?

Should each such page have its own selection mechanism? Or should we have a (set of) selection page(s) which the user should visit first, saving selection to session state, using session state on other pages to visualize?

The second option would avoid duplicating all the selection code (keeping in mind that streamlit will do a lot of re-running on individual pages if there's a bunch of clicking to narrow down selections). On the other hand, we'd need to somehow convey that some pages need you to go to "choose single CCV ID" page, others would need "choose CCV IDs" page.

This of course relates to #1