expand a quarterly series into a monthly one
tsout=q2m(tsin)
* tsin = a quarterly time series
* option = an integer, between -1 and 12:
. -1 if each monthly value is equal to the quarterly one (default if the argument is not given)
. 0 if each monthly value is equal to the quarterly one divided by 3
. n if the value of the n-th month is equal to the quarterly one and the others are set to %nan
* tsout = the corresponding monthly time series