get_by_set_pos
Description:
Gets the by_set_pos value at index index.
The index should be less than i_cal_by_setpos_size.
Parameters:
| this | The Recurrence | 
| index | The index in by_set_pos of Recurrence, less than i_cal_by_setpos_size | 
Returns:
| The by_week_no of Recurrence at index  |