FEM: Use single smesh mesh generator throughout FreeCAD

Creating a mesh generator resets a critical data structure and makes all existing meshes invalid. Hence the SMESH_gen is made a singleton and all FreeCAD code is changed accordingly.

src/3rdParty/salomesmesh/inc/SMESH_Gen.hxx

@@ -60,8 +60,9 @@ typedef std::set<int> TSetOfInt;
 class SMESH_EXPORT  SMESH_Gen
 {
 public:
-  SMESH_Gen();
   ~SMESH_Gen();
+  
+  static SMESH_Gen* get();
 
   SMESH_Mesh* CreateMesh(int theStudyId, bool theIsEmbeddedMode)
     throw(SALOME_Exception);
@@ -159,6 +160,10 @@ public:
   // std::map < int, SMESH_2D_Algo * >_map2D_Algo;
   // std::map < int, SMESH_3D_Algo * >_map3D_Algo;
 
+protected:
+  SMESH_Gen();
+  static SMESH_Gen* generator;
+  
 private:
 
   int _localId;                         // unique Id of created objects, within SMESH_Gen entity

src/3rdParty/salomesmesh/src/SMESH/SMESH_Gen.cpp

@@ -55,6 +55,7 @@ using namespace std;
 
 //#include <vtkDebugLeaks.h>
 
+SMESH_Gen* SMESH_Gen::generator = nullptr;
 
 //=============================================================================
 /*!
@@ -74,6 +75,15 @@ SMESH_Gen::SMESH_Gen()
   //vtkDebugLeaks::SetExitError(0);
 }
 
+SMESH_Gen* SMESH_Gen::get() {
+
+    if(!generator)
+        generator = new SMESH_Gen();
+    
+    return generator;
+}
+
+
 //=============================================================================
 /*!
  * Destructor