Add Antarctic Oscillation

[eliocamp]

Since the package can download the Arctic Oscillation index, it stands to reason to add also its Southern Hemisphere sibling. The data here:

https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/history/method.shtml

[boshek]

Adding this table here as it took me a little time to find it: https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/monthly.aao.index.b79.current.ascii.table