Shift manuscript/supplemental asset creation to wrapper

[jcoliver]

The two files creating objects for publication, create-manuscript-objects.R and create-supplement-objects.R (both in src/summary) could be revised into wrappers that call individual scripts that create each of those objects. Would be a good opportunity to see what assets we want and how to categorize them. Some objects may have shared data, and sometimes shared data that are wrangled (away) from the version on disk. This may prompt further separation of the creation of the data that are used for those objects and the creation of the objects themselves.