fix usage of dataMap and simplify,document aggregate()
fix usage of dataMap and simplify,document aggregate()

raintank/metrictank,replay/metrictank,raintank/raintank-metric,replay/metrictank,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/raintank-metric,raintank/metrictank,raintank/metrictank,raintank/raintank-metric