mber to April as high flow season.

   INPUT
   X: time series - should be continuous (e.g. Q, typically [mm/timestep])
   t: time [Matlab datetime]
   subperiod: subperiod to be extracted, e.g. [11:12, 1:4] for November
       to April or [5:9] for May to September
   OPTIONAL
   option: option to delete values outside subperiod ('delete') or assign
       NaN to values outside subperiod ('nan')

   OUTPUT
   X_sub: subperiod time series
   t_sub: subperiod time

   EXAMPLE
   % load example data
   data =