Creates a new Aggregator object.
Arguments
- label
a human readable label
- aggregate
a function with argument
evthat 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 anEvalsobjecto,aggregatetakes as inputo@evals[[method_name]](which is a list of the kind just described). The function aggregate should return a scalar.