[Gecode] Simplified commit

[Christian Schulte]

The simplification takes into account that the preferred way to do search in
Gecode is based on batch recomputation.

A branching now only requires a commit(int,BranchDesc*) virtual member
function. This member function must be prepared to be called with NULL as
BranchDesc in which case commiting has to be driven by information taken
from the branching.

All code has been adapted (I also made branching for sets more incremental).

Christian 

--
Christian Schulte, http://www.imit.kth.se/~schulte/