Review Handle class

I've changed
   1. getLink(...) to currentLink(...) and
   2. setLink(...) to linkTo(...)

These changes will make classes Handle and Link look closer to QuantLib/C++ but there are still structural differences.

I've tried to removed "implements Observable" from class Handle and move all responsibilities to class Link but it didn't not work. A new tentative is needed after I figure out what would be the responsibilities of Link in this new scenario (I mean: in the original scenario defined by Quantlib/C++).

These comments where taken from the new implementation of Handle:
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In QL/C++, Handle has operators which return the inner Link class, performing a call to currentLink() behind the scenes. This is pretty handy from the calling code because you can simply dereference a Handle and (like magic!) call methods from the parameterized class.

In Java we are obliged to explicitly call currentLink() in order to do the same thing.

The difficulty arises when we need to register Observers.

In the current implementation, we hide class Link from outside world and we only expose the Handle itself, which implements Observable. Doing this way, the calling Java code does not need to decide which object is really the Observer object because class Handle is responsible for hiding this implementation details from outside world and properly forwarding notifications to external Observers as expected.

done