Deprecate RelinkableHandle constructor taking a raw pointer

[sweemer]

I'm getting 'QuantLib::Handle<T>::Handle': no overloaded function could convert all the argument types when trying to use the constructor of RelinkableHandle that takes a raw pointer. Not sure if I'm doing it right, but even if it does work, I believe that this constructor should be deprecated for the following reasons:

• Handle does not provide a constructor taking a raw pointer and it's unclear why RelinkableHandle needs to.
• shared_ptr takes ownership of the raw pointer, which may not be obvious from the constructor declaration, and can lead to double delete bugs.

Also I changed a couple of places to pass by value and move.

[coveralls]

coverage: 72.55% (-0.001%) from 72.551%
when pulling a19e2ec on sweemer:relinkable-handle
into b981510 on lballabio:master.

[sweemer]

@lballabio Do you want me to revert these back to const reference too? Not sure whether you think that the case we want to optimize for is the same between shared_ptr and Handle.

[lballabio]

Hmm. We're dealing with just Handle and RelinkableHandle here, not a bazillion indexes, so we might consider overloading on const T& and T&&. What do you think?

[sweemer]

Sounds good. I'll add those for both Handle and RelinkableHandle.

[sweemer]

I added a default constructor for RelinkableHandle to disambiguate between the two new constructors, and also changed linkTo to pass by value, both of which are now in line with Handle.