Do edge swapping for faces individually with tensor product meshes

If the mesh contains quads, the edge swapping algorithm switches to
generic improve, which introduces quads everywhere. This is not intended
if one domain contains a tensor product mesh.
Thus, call the optimizer for each face if mesh contains quads but mp.quad
is not set.

libsrc/meshing/meshfunc2d.cpp

@@ -20,6 +20,19 @@ namespace netgen
       }
     mesh.Compress();
 
+    bool optimize_swap_separate_faces = false;
+    if(!mp.quad)
+    {
+      bool mixed = false;
+      ParallelFor( Range(mesh.GetNSE()), [&] (auto i) NETGEN_LAMBDA_INLINE
+          {
+            if (mesh[SurfaceElementIndex(i)].GetNP() != 3)
+                mixed = true;
+          });
+      if(mixed)
+        optimize_swap_separate_faces = true;
+    }
+
     const char * optstr = mp.optimize2d.c_str();
     int optsteps = mp.optsteps2d;
 
@@ -33,14 +46,40 @@ namespace netgen
 	      {  // topological swap
                 MeshOptimize2d meshopt(mesh);
                 meshopt.SetMetricWeight (mp.elsizeweight);
+
+                if(optimize_swap_separate_faces)
+                {
+                  for(auto i : Range(1, mesh.GetNFD()+1))
+                  {
+                    meshopt.SetFaceIndex(i);
                     meshopt.EdgeSwapping (0);
+                  }
+                }
+                else
+                {
+                  meshopt.SetFaceIndex(0);
+                  meshopt.EdgeSwapping (0);
+                }
 		break;
 	      }
 	    case 'S': 
 	      {  // metric swap
 		MeshOptimize2d meshopt(mesh);
                 meshopt.SetMetricWeight (mp.elsizeweight);
+
+                if(optimize_swap_separate_faces)
+                {
+                  for(auto i : Range(1, mesh.GetNFD()+1))
+                  {
+                    meshopt.SetFaceIndex(i);
                     meshopt.EdgeSwapping (1);
+                  }
+                }
+                else
+                {
+                  meshopt.SetFaceIndex(0);
+                  meshopt.EdgeSwapping (1);
+                }
 		break;
 	      }
 	    case 'm':