Fix https://github.com/julianhyde/optiq/issues/216, "Inconsistent use of provided operator table causes inability to add aggregate functions".

Now you can specify via the Frameworks.getPlanner interface a SqlOperatorTable that contains a user-defined aggregate function, and the query will validate appropriately.

Added a test case for https://github.com/julianhyde/optiq/issues/217, "Validator should reject non-aggregate expressions in SELECT clause with aggregates".