Extract Splitter and Parameter class.
Refactoring parallel fold using splitter.
Implements chunk splitter.
Optimize chunk splitter.
Add parameter to select splitter.
Implements into operator that supports parallel execution.
Implements toList operator that supports parallel execution.
Fix skip test.
Consolidate fold test.
Rename test cases of parallel test.
Fix test that fail on parallel execution.
Implements map operator that supports parallel execution.
Separate grouping operators to non-parallel and parallel.