Remove agg panics
Remove agg panics

If 0 length is passed in, we should return math.NaN like python code
Also follow SEP (Somebody else's problem)

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