Multiple improvement (#161,#160,#159,#146)

[Fix #161] Propagator: fine_evt and default implementation of propagate(int,int)
[Fix #160] speed up count propagator
[Fix #159] avoid stackoverflow using GCC_fast
[Fix #146] a new propagation engine is now available which manages coarse propagations
[Fix #146] a new propagation engine is now available which manages coarse propagations

The TwoBucketsPropagationEngine is now the default one. It manages both priority and coarse propagators in an efficient way.
There are still little improvements to be done.

A new method is now mandatory in IPropagationEngine: delayedPropagation().
Remove VariableEngine and PropagatorEngine
Remove eventmasks from Propagator
Add a test to compare the two propagation engines
Merge branch 'develop' into feature-pren
Merge branch 'feature-pren' into develop