Fix Version/s: None
Implement subset and merge on SqliteRegistry (mostly on SqlRegistry, actually) that subsets and merges the contents of multiple Collections into a single Collection.
The arguments should probably be similar or the same as those to makeDataGraph; we want to allow any of the following criteria:
- a list of Collections to draw from (searched in the specified order)
- a list of DatasetTypes to consider
- a user expression of the same form accepted by makeDataGraph
Subset has been at least mostly implemented on
DM-17023; merge (which involves ID reassignment in general) is what remains. Adjusting title accordingly.