FEM: elmer writer, only write denisty if really needed, partitially revert 2d73444a66, unit test fix in separate commit

src/Mod/Fem/femsolver/elmer/writer.py

@@ -636,11 +636,24 @@ class Writer(object):
         return None
 
     def _handleElasticityMaterial(self, bodies):
+        # density
+        # is needed for self weight constraints and frequency analysis
+        density_needed = False
+        for equation in self.solver.Group:
+            if femutils.is_of_type(equation, "Fem::EquationElmerElasticity"):
+                if equation.DoFrequencyAnalysis is True:
+                    density_needed = True
+                    break  # there could be a second equation without frequency
+        gravObj = self._getSingleMember("Fem::ConstraintSelfWeight")
+        if gravObj is not None:
+            density_needed = True
+        # temperature
         tempObj = self._getSingleMember("Fem::ConstraintInitialTemperature")
         if tempObj is not None:
             refTemp = self._getFromUi(tempObj.initialTemperature, "K", "O")
             for name in bodies:
                 self._material(name, "Reference Temperature", refTemp)
+        # get the material data for all boddies
         for obj in self._getMember("App::MaterialObject"):
             m = obj.Material
             refs = (
@@ -649,12 +662,11 @@ class Writer(object):
                 else self._getAllBodies()
             )
             for name in (n for n in refs if n in bodies):
-                # density has to be written even without self weight constraint
+                if density_needed is True:
-                # https://forum.freecadweb.org/viewtopic.php?f=18&t=56590#p487117
+                    self._material(
-                self._material(
+                        name, "Density",
-                    name, "Density",
+                        self._getDensity(m)
-                    self._getDensity(m)
+                    )
-                )
                 self._material(
                     name, "Youngs Modulus",
                     self._getYoungsModulus(m)