Opening of usable functions in SeriesAggreg

"min", "max" and "median" should be usable besides "sum" and "mean" functions currently available in SeriesAggreg. But also other functions as below:

Quantile (percentile)

A call to a function named "qn" should be interpreted as quantile(x, probs = n) (i.e. "q90" as probs = 0.9)

Any function taking a vector as first argument and returning a single value.

Edited Jan 08, 2021 by Dorchies David