transforms monthly data into annual ones
tsout=m2a(tsin,ind)
* tsin = a monthly timeseries
* ind =
. -1 if the annual data is to be the sum of the corresponding months
. 0 if the annual data is to be the mean of the corresponding months
. i if the annual value is to be eqaul to the value of the i-th month
ind is optional; if not provided, it is set to -1
* tsout = the corresponding annual time series