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