Note: DM-15214 adds some custom ordering code for DataUnits that should be obsoleted and replaced by this ticket.

Andy Salnikov, just a heads up that I'm starting on this now, and it'll involve a pretty thorough rewrite of the APIs for workings with DataUnits and DataUnitJoins, and hence it'll involve some big changes in the preflight code as well.

I'll ping you again once I start working on the changes to preflight, as we probably want to avoid us both making changes there at the same time (but I'm not there yet).

This big ticket is ready for review, and  Andy Salnikov, I'm afraid I need to ask you to be the on to look at most of it (the changes in daf_butler), even though I already asked you to look at the first draft of some of this a week ago.

Christopher Waters, could you take a look at the downstream changes to obs_base, skymap, pipe_base, pipe_tasks, and ci_hsc?  Only the first of those has more than trivial renames, and one specific question I have for you is whether the logic there is big-picture understandable without having to look at the docs for the new DataId object in daf_butler.

Jonathan Sick, if you have time to look at the docstrings in daf.butler.dimensions I'd be grateful; there are a few things (**kwds arguments, plurals of symbols) in particular that I'm not sure I'm handling appropriately.

I'd previously forgotten about pipe_supertask, which also needed some (happily trivial) changes.  Now included as well.

I checked daf_butler, looks OK in general and I think I managed to understand how it works  Small bunch of comments on PR. I have also approved pipe_base and pipe_supertask. Removing myself from reviewers, Christopher Waters, please mark it as reviewed once you are done with remaining packages.

I've looked through the set excluding `daf_butler` (and also enough of `daf_butler` to convince myself I vaguely know how this works).  I think the `obs_base` code is generally understandable, although I've added comments on the pull request where I was either confused or unclear.  There are a handful of other small comments (typos and such).

Merged to master.

Thanks, all, for the fast and very helpful reviews.