Expands an annual series into a monthly one, by duplicating 12 times the annual values.
EXAMPLE
x=reshape([1:35],'1978a')// ceate an annual tsy=a2m(x)// gives an montly ts, from 1978m1 to 2012m12 with values 1 for all months of year 1978, // 2 for all months of year 1979...