Sketcher: Solver Debug functionality: Ability to export a subsystem

===================================================================

It allows to export the c++ code to create a subsystem with the same information as the one solved using LM/DL/BGFS.

In this commit the functionality is disabled (for production).

To enable the functionality uncomment this line in planegcs/Constraints.h:
//#define _GCS_EXTRACT_SOLVER_SUBSYSTEM_

When enabled, upon solving with LM/DL/BGFS, the c++ code to generate the subsystem is added to a subsystem.txt that is created in the FreeCAD
execution directory.

Note that the file is created in append mode, so it will append all normal/redundant solvings until the file is deleted.

The resulting code can be directly pasted into a project similar to:
https://github.com/abdullahtahiriyo/Eigen_LUPiv_Convergence

Such a project only has libeigen as external dependency.

src/Mod/Sketcher/App/planegcs/SubSystem.h

@@ -63,6 +63,8 @@ namespace GCS
         void getParams(Eigen::VectorXd &xOut);
         void setParams(VEC_pD &params, Eigen::VectorXd &xIn);
         void setParams(Eigen::VectorXd &xIn);
+        
+        void getConstraintList(std::vector<Constraint *> &clist_);
 
         double error();
         void calcResidual(Eigen::VectorXd &r);