FEM: Add support for CalculiX truss elements (#23224)

* FEM: Update element_geometry1D.py

* FEM: Update write_femelement_geometry.py

* FEM: Update write_mesh.py

* FEM: Update solver.py

* FEM: Update solver_calculix.py

* FEM: Update element_geometry1D.py

src/Mod/Fem/femobjects/element_geometry1D.py

@@ -175,7 +175,16 @@ class ElementGeometry1D(base_femelement.BaseFemElement):
                 value=["Rectangular", "Circular", "Pipe", "Elliptical", "Box"],
             )
         )
-
+        prop.append(
+            _PropHelper(
+                type="App::PropertyArea",
+                name="TrussArea",
+                group="TrussSection",
+                doc="Set cross-sectional area of truss elements\n"
+                + "(used if bending stiffness is excluded in the solver)",
+                value=10.0,
+            )
+        )
         return prop
 
     def onDocumentRestored(self, obj):

src/Mod/Fem/femobjects/solver_calculix.py

@@ -326,7 +326,7 @@ class SolverCalculiX(base_fempythonobject.BaseFemPythonObject):
                 type="App::PropertyBool",
                 name="ExcludeBendingStiffness",
                 group="Solver",
-                doc="Exclude bending stiffness to replace shells with membranes",
+                doc="Exclude bending stiffness to replace shells with membranes or beams with trusses",
                 value=False,
             )
         )