Creates a new Aggregator object.
new_aggregator(label, aggregate)a human readable label
a function with argument ev that is a list of length
equal to the number of draws with each element itself being a named list.
Each element of this list corresponds to a metric that has been computed.
In particular, given an Evals object o,
aggregate takes as input o@evals[[method_name]] (which is a
list of the kind just described). The function aggregate should return a
scalar.