This PR adds the functionality to combine multiple databases in one interface.
Specifically, the new DatabaseMerger filter takes two tables as its input and produces a combined table based via a SQL full inner join on shared columns.
Then this 'master table' can be used to generate parameter widgets that generate SQL statements for multiple databases.
An example is provided in examples/database_compositing_pipeline.ipynb which combines two databases of the pycinema-data repository.
DatabaseMergerfilter takes two tables as its input and produces a combined table based via a SQL full inner join on shared columns.examples/database_compositing_pipeline.ipynbwhich combines two databases of thepycinema-datarepository.