Fix https://github.com/julianhyde/optiq/pull/227, "Extend extract function to support DateTime data types".
Extend PlannerImpl to pass a custom ConvertletTable.

Add test to SqlOperatorBaseTest.
Use a singleton in StandardConvertletTable.
Implement EXTRACT for DATE, TIME and TIMESTAMP values.
Fix julian date formula for dates before 1900-03-01.
Convert CombinerParser into freemarker template to allow custom parser statements