Performance improvement for GET_ROW/GET_COLUMN

[Pacojka]

Hallo Ivan,

i implemented the changes as described in issue #528 for rows and columns objects.

I kept the object_collection and the get_iterator method because lack of time and the visibility (public) of the get_iterator method.

In the worksheet class, I changed get_row and get_column.

[Pacojka]

I tested the change using ZDEMO_EXCEL15. Everything looks fine.
The performance improvement is as expected.

[ivanfemia]

Thanks.
I think that we should improve this removing completely the iterator and the few methods no longer needed... But first of all we need to mark those as obsolete