TDD commit of UDA unit test
Refactor stateful apply implementation for clarity
Factor out update, initialization logic
Add state variables to the groupby operator
Support UDA-type aggregates in fakedb
Test of fakedb uda functionality
Rename AggregateExpression BuiltinAggregateExpression
Get rid of redundant is_aggregate function
Add comment
Create a UdaAggregateExpression class
Use AggregateExpression for aggregate type checks
Plumb statemods into groupby
Add UDAs to the myrial grammer
Fix grammer in UDA test case
Wrap UDA expressions in a UdaAggregateExpression
Add comment for GroupBy
Use state_schema to compute GroupBy output schema
Exempt UDAs from simple group by logic
Handle non-decomposable aggregates in the optimizer
Add statemods to groupby repr