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.