add optimality gap as termination criterion to gurobi solver

NSolver/GUROBISolver.cc

@@ -97,7 +97,8 @@ GUROBISolver::
 solve(NProblemInterface*                  _problem,
       const std::vector<NConstraintInterface *> &_constraints,
       const std::vector<PairIndexVtype> &_discrete_constraints,
-      const double                        _time_limit)
+      const double                        _time_limit,
+      const double                        _gap)
 {
   DEB_enter_func;
   try
@@ -237,6 +238,8 @@ solve(NProblemInterface*                  _problem,
         GurobiHelper::outputModelToMpsGz(model, problem_output_path_);
       }
 #endif//COMISO_QT_AVAILABLE
+      if (_gap > 0.0)
+        model.getEnv().set(GRB_DoubleParam_MIPGap, _gap);
       model.optimize();
     }
     else

NSolver/GUROBISolver.hh

@@ -46,7 +46,9 @@ public:
   bool solve(NProblemInterface*                  _problem,                      // problem instance
              const std::vector<NConstraintInterface*>& _constraints,            // linear constraints
              const std::vector<PairIndexVtype>&        _discrete_constraints,   // discrete constraints
-             const double                        _time_limit = 60     );        // time limit in seconds
+             const double                        _time_limit = 60,              // time limit in seconds
+             const double                        _gap = 0.0);                   // stops when solution with optimality gap
+                                                                                // lower than _gab is reached
 
   bool solve(NProblemInterface*                  _problem,                // problem instance
              const std::vector<NConstraintInterface*>& _constraints,      // linear constraints