FEM: gmsh, add linear nodes interpolation parameter (thanks to _UR)

src/Mod/Fem/femmesh/gmshtools.py

@@ -757,6 +757,16 @@ class GmshTools():
             geo.write("Coherence Mesh; // Remove duplicate vertices\n")
         else:
             geo.write("Mesh  " + self.dimension + ";\n")
+        if hasattr(self.mesh_obj, "SecondOrderLinear") and self.mesh_obj.SecondOrderLinear is True:
+            geo.write(
+                "Mesh.SecondOrderLinear = 1; // Second order nodes are created "
+                "by linear interpolation instead by curvilinear\n"
+            )
+        else:
+            geo.write(
+                "Mesh.SecondOrderLinear = 0; // Second order nodes are created "
+                "by linear interpolation instead by curvilinear\n"
+            )
         geo.write("\n")
 
         # save mesh

src/Mod/Fem/femobjects/_FemMeshGmsh.py

@@ -174,6 +174,14 @@ class _FemMeshGmsh():
         )
         obj.GeometryTolerance = 1e-06
 
+        obj.addProperty(
+            "App::PropertyBool",
+            "SecondOrderLinear",
+            "FEM Gmsh Mesh Params",
+            "Second order nodes are created by linear interpolation"
+        )
+        obj.SecondOrderLinear = True
+
         obj.addProperty(
             "App::PropertyEnumeration",
             "Algorithm2D",