Operations Aggregate, Average and Sum with selector

[akarnokd]

Issue #653

Remarks:

• I know we can combine ops to get one of the new aggregate variant, but I think it might be worth having a direct version which avoids nesting several layers of Observables, Observers and Subscriptions.
• The averageInteger and sumInteger (and the other types) are handy if we want to use chained operation invocations (with less overhead):

Observable.from("a", "bb", "ccc").sumInteger(s -> s.length())
    .toBlockingObservable().single();

instead of

Observable.sumIntegers(Observable.from("a", "bb", "ccc").map(s -> s.length()))
    .toBlockingObservable().single();

[cloudbees-pull-request-builder]

RxJava-pull-requests #590 SUCCESS
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[benjchristensen]

This establishes (or strengthens) a precedent of adding a large number of helper methods for very specific cases to the already massive Observable so I'd like to explore one of two routes:

1. Move mathematical operators to a separate Observable in rx.observable.NumericalObservable or something like that.
2. Move them into a contrib module like https://github.com/Netflix/RxJava/blob/master/rxjava-contrib/rxjava-string/src/main/java/rx/observables/StringObservable.java

[akarnokd]

I'd go for NumericalObservable on this one, as it is primarily useful out-of-box whereas Futures may go into contrib.

[benjchristensen]

This is equivalent to:

reduce(seed, accumulator).map(resultSelector)

Why the need for a separate operator and implementation?

[akarnokd]

I know we can combine ops to get one of the new aggregate variant, but I think it might be worth having a direct version which avoids nesting several layers of Observables, Observers and Subscriptions.

[benjchristensen]

nesting several layers of Observables, Observers and Subscriptions.

The entire library is based on composition. If we were seeking to avoid that we wouldn't be using a functional style so unless there is a strong performance reason (that isn't a bug with one of the operators themselves) I don't see that as a reason to add a helper operator.

[akarnokd]

Okay.