Get all study or population items (e.g., geneids).

def get_item_cnt(results, attrname="study_items"):
        """Get all study or population items (e.g., geneids)."""
        items = set()
        go_cnt = 0
        for rec in results:
            if hasattr(rec, attrname):
                items_cur = getattr(rec, attrname)
                # Only count GO term if there are items in the set.
                if len(items_cur) != 0:
                    items |= items_cur
                    go_cnt += 1
        return items, go_cnt