transforms monthly data into quarterly ones
tsout=m2q(tsin,ind)
* tsin = a monthly timeseries
* ind =
- -1 if the quarterly data is to be the sum of the corresponding months
- 0 if the quarterly data is to be the mean of the corresponding months
- 1 if the quarterly data is to be the value of the first month
- 2 if the quarterly data is to be the value of the second month
- 3 if the quarterly data is to be the value of the third month
ind is optional; if not provided, it is set to -1
* tsout = the corresponding quarterly time series