subper — restrict a ts to a subperiod
tsout=subper(tsin,arg1,[arg2])
tsin = a timeseries
arg1= either a (1 x 2) vector of dates strings or a dates strings, representing the begining of the subperiod
arg2= (if arg1 was a dates string), a dates string, representing the end of the subperiod
tsout = the timeseries over the subperiod