Tim Jenness, could you please review the daf_butler, obs_base, pipe_tasks, and obs_subaru changes?

Andy Salnikov, could you please review the pipe_base and ctrl_mpexec changes?

Most changes are in daf_butler, with a fair amount in pipe_base.  Everything else is trivial and repetitive.

The only change visible outside daf_butler is that DatasetType can now only be constructed with normalized dimensions (i.e. a DimensionGraph) and that means pipe_base's DatasetTypeDescriptor now holds the unnormalized components of a DatasetType instead of holding a DatasetType directly.

I reviewed pipe_base and ctrl_mpexec, looks OK. Removing myself from reviewers.

I approve with a heavy heart. I'm somewhat disappointed at how much extra overhead is now involved in testing even the simplest thing.

Merged to master.  Tim Jenness, I'll look for ways to restore some of the testing flexibility we previously had in future refactors - I think I see a path to allowing new trivial dimensions to be added as long as they don't change existing relationships, for example (though it may take me a while to get to that).