Gecode 5.1.0

This is a rather major release, fixing a number of bugs and adding quite a number of new features. Some of the features require changes to your models.

In more detail: extended tracing so that all propagator executions and commit operations can be traced; renamed activity to action to avoid confusion with activity in SAT (and fixed that action information is also recorded during failure); added CHB (conflict-history based branching) heuristic; optimized AFC; all functions taken as arguments by Gecode are now based on std::function-types; instead of branching with INT_VAR_* and INT_VAL_* for Boolean variables, BOOL_VAR_* and BOOL_VAL_* must be used. There are a number of additional changes, so you might want to read the whole changelog.