Changed GecodeSpace propagation mechanics (major).
- Added a thread to run propagation. The thread is added to the space whenever a constraint is posted over constraint variables.
- Removed temporal code from Space.ask in ReifiedSpace. The code to check whether the space is failed or distributable w.r.t the GecodeSpace is now in checkStability of the mozart space.
- The commit operation has changed to use Gecode::rel instead of Gecode::commit. This breaks temporarily the support for variable and selection strategies of Gecode.
- Still to improve: execute propagation once and only once in each space.