Creates an iterator to iterate through the filtered data. By default, 100 tuples are cached. 

The identifier is cleaned up upon creation of the iterator. 

Also initializes the iterator: Moves the current position to before the beginning of the data; a subsequent next call will find the first non-default position.

virtual class IIterator* createIterator(IFilter* filter, 
                                                int tuplesAhead = 100) = 0;