Create a public CalculationListener returning a future

[cjkent]

This PR renames AggregatingListener and moves it to the top level so it can be used by external code.

Having a CalculationListener returning a CompletableFuture is essential for writing fully asynchronous code with Strata. It seems a shame that users will have to implement it themselves when we already have one in Strata.

[jodastephen]

Approved, as there is nothing wrong with this. However, the new public class does seem a little isolated. Another problem is that the user has is they need to pass in the columns to the ResultsListener (and typically other Strata classes have factory methods). Not sure there is a way to setup the columns automatically, but it would be nice. Maybe a calculationStarted(columns) method on CalculationListener ?

[cjkent]

I agree that it's not particularly elegant to pass the columns to the listener and the calculation runner. I like your suggestion. I'll merge this PR and raise a follow up to add calculationStarted()