Deferred Observable Subscription

[stealthcode]

Delay the subscription of each subscription in a DualOperator until the observable needs to be subscribed to. For instance:

Observable<Integer> nums;
DyadObservable.sparseProduct(nums, this::factorsOfInteger)
    .filter1((Integer i) -> { return i % 2 != 0; })
    .bimap((Integer root, Integer factor) -> { 
        return root + " : " + factor;
    })
    .subscribe();

The method Observable<Integer> factorsOfInteger(Integer) should never be called for any even integer.