Gecode support for FlatZinc modelling language

 Gecode is a toolkit for developing constraint-based systems and
 applications. Gecode provides a constraint solver with
 state-of-the-art performance while being modular and extensible.
 .
 FlatZinc is a low-level modelling language for constraint
 problems. It is designed to be easily interfaceable to constraint
 solvers (like Gecode). For more information on FlatZinc, please refer
 to the MiniZinc pages of the G12 project <https://minizinc.org/>.