Combine a series with another time series

Climate indices are often defined as the (normalised) difference
between two time series. Examples are common definitions of the NAO
index and the SOI. This options allows you to subtract two time series of your
own choosing to form a new one.
Note that there are many different ways to normalise. First the anomalies are computed with respect to te annual cycle. Next the series are optionally normalised to standard deviation one, either for each calendar month separately or everyting together. Finally the difference of these series is optionally normalised to standard deviation one, again either for each month separately or just the whole series.