Update QueryEngine API to support multiple results tables #2363
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In future Datasets will be able to produce more than one results table and so this is the API we need.
If you've got a single table to read then it's obviously sensible to ask to read this from e.g. a single CSV file. But if you've got multiple tables to read then you'll need to supply e.g. a directory of CSV files. Similarly for writing data. Handling this logic inside the functions here simplifies things elsewhere.
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This PR refactors the query engine API and the plumbing around it such that it could in principle support returning event-level data if only we had some datasets which returned more than one query.
The major change to the query engine API is replacing the
get_results()method, which returned an iterator over a single table of results, withget_results_tables()which returns a nested iterator over multiple tables of results.To avoid massive changes to the test suite (and particularly to the under-development dummy data tests) we retain a
get_results()method which asserts that we only have a single table of results and returns it as previously.The SQL engines also change their
get_query()method toget_queries()to support multiple queries.