get_by_year_day
Description:
Gets the by_year_day value at index index.
The index should be less than i_cal_by_yearday_size.
Parameters:
| this | The Recurrence | 
| index | The index in by_year_day of Recurrence, less than i_cal_by_yearday_size | 
Returns:
| The by_year_day of Recurrence at index  |