Aligns several TimeSeries, so that their starting and ending dates match.
Series are resized and filled with masked values accordingly.
The resulting series have no missing dates (ie. series.is_valid() == True for each of the resulting series).
The function accepts two extras parameters: - start_date forces the series to start at that given date, - end_date forces the series to end at that given date.
By default, start_date and end_date are set respectively to the smallest and largest dates of the series.