get_by_month_day
Description:
Gets the by_month_day value at index index.
The index should be less than i_cal_by_monthday_size.
Parameters:
| this | The Recurrence | 
| index | The index in by_month_day of Recurrence, less than i_cal_by_monthday_size | 
Returns:
| The by_month_day of Recurrence at index  |