[Gecode] Another abstraction sets...

[Gabor Szokoli]

Christian Schulte wrote:

>...how about having card() (or size() as it is called for integers). A look
>to branch.cc makes it obvious.
>  
>
Hello!

We used to have methods for accessing the "unknown" set as an iterator, 
upper minus lower bound, but we only needed it in the distributors, so 
it became a victim of interface unbloating.

Getting its size was of course uboptimal, as it equals ubSize()-lbSize().
I fixed only this now.

Gabor Szokoli