[FEM] add support for 3D recombinations (#4706)

* [FEM] add support for 3D recombinations

currently we only support surface recombinations but for some applications 3D recombinations are useful as well

* add support for the recombination algorithms

using a sensible algorithm is important to get useful results, see https://wiki.freecadweb.org/FEM_MeshGmshFromShape#Properties
where I described examples

src/Mod/Fem/femmesh/gmshtools.py

@@ -125,6 +125,19 @@ class GmshTools():
         else:
             self.algorithm3D = "1"
 
+        # RecombinationAlgorithm
+        algoRecombo = self.mesh_obj.RecombinationAlgorithm
+        if algoRecombo == "Simple":
+            self.RecombinationAlgorithm = "0"
+        elif algoRecombo == "Blossom":
+            self.RecombinationAlgorithm = "1"
+        elif algoRecombo == "Simple full-quad":
+            self.RecombinationAlgorithm = "2"
+        elif algoRecombo == "Blossom full-quad":
+            self.RecombinationAlgorithm = "3"
+        else:
+            self.algoRecombo = "0"
+
         # HighOrderOptimize
         optimizers = self.mesh_obj.HighOrderOptimize
         if optimizers == "None":
@@ -766,8 +779,15 @@ class GmshTools():
             )
         geo.write("\n")
         if hasattr(self.mesh_obj, "RecombineAll") and self.mesh_obj.RecombineAll is True:
-            geo.write("// other mesh options\n")
+            geo.write("// recombination for surfaces\n")
             geo.write("Mesh.RecombineAll = 1;\n")
+        if hasattr(self.mesh_obj, "Recombine3DAll") and self.mesh_obj.Recombine3DAll is True:
+            geo.write("// recombination for volumes\n")
+            geo.write("Mesh.Recombine3DAll = 1;\n")
+        if ( (hasattr(self.mesh_obj, "RecombineAll") and self.mesh_obj.RecombineAll is True)
+             or (hasattr(self.mesh_obj, "Recombine3DAll") and self.mesh_obj.Recombine3DAll is True)):
+            geo.write("// recombination algorithm\n")
+            geo.write("Mesh.RecombinationAlgorithm = " + self.RecombinationAlgorithm + ";\n")
             geo.write("\n")
 
         geo.write("// optimize the mesh\n")

src/Mod/Fem/femobjects/mesh_gmsh.py

@@ -60,6 +60,12 @@ class MeshGmsh(base_fempythonobject.BaseFemPythonObject):
         "R-tree",
         "HXT"
     ]
+    known_mesh_RecombinationAlgorithms = [
+        "Simple",
+        "Blossom",
+        "Simple full-quad",
+        "Blossom full-quad"
+    ]
     known_mesh_HighOrderOptimizers = [
         "None",
         "Optimization",
@@ -169,7 +175,7 @@ class MeshGmsh(base_fempythonobject.BaseFemPythonObject):
                 "App::PropertyBool",
                 "OptimizeStd",
                 "FEM Gmsh Mesh Params",
-                "Optimize tetra elements"
+                "Optimize tetrahedral elements"
             )
             obj.OptimizeStd = True
 
@@ -201,6 +207,25 @@ class MeshGmsh(base_fempythonobject.BaseFemPythonObject):
             )
             obj.RecombineAll = False
 
+        if not hasattr(obj, "Recombine3DAll"):
+            obj.addProperty(
+                "App::PropertyBool",
+                "Recombine3DAll",
+                "FEM Gmsh Mesh Params",
+                "Apply recombination algorithm to all volumes"
+            )
+            obj.Recombine3DAll = False
+
+        if not hasattr(obj, "RecombinationAlgorithm"):
+            obj.addProperty(
+                "App::PropertyEnumeration",
+                "RecombinationAlgorithm",
+                "FEM Gmsh Mesh Params",
+                "Recombination algorithm"
+            )
+            obj.RecombinationAlgorithm = MeshGmsh.known_mesh_RecombinationAlgorithms
+            obj.RecombinationAlgorithm = "Simple"
+
         if not hasattr(obj, "CoherenceMesh"):
             obj.addProperty(
                 "App::PropertyBool",