Converts the dates to a ndarray of strings.
The format of the strings depends of the frequency of the instance.
Examples
>>> d = ts.date_array(start_date=ts.Date('M', '2001-01'), length=5)
>>> d.tostrings()
array(['Jan-2001', 'Feb-2001', 'Mar-2001', 'Apr-2001', 'May-2001'],
dtype='|S8')