FEM: Add accuracy parameter for CalculiX buckling analysis (#18790)

* FEM: Update solver.py

* FEM: Update write_step_equation.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/Mod/Fem/femsolver/calculix/solver.py

@@ -316,6 +316,15 @@ class _BaseSolverCalculix:
             )
             obj.ThermoMechType = thermomech_types
 
+        if not hasattr(obj, "BucklingAccuracy"):
+            obj.addProperty(
+                "App::PropertyFloatConstraint",
+                "BucklingAccuracy",
+                "Fem",
+                "Accuracy for buckling analysis",
+            )
+            obj.BucklingAccuracy = 0.01
+
 
 class Proxy(solverbase.Proxy, _BaseSolverCalculix):
     """The Fem::FemSolver's Proxy python type, add solver specific properties"""

src/Mod/Fem/femsolver/calculix/write_step_equation.py

@@ -154,7 +154,10 @@ def write_step_equation(f, ccxwriter):
             ccxwriter.solver_obj.TimeMaximumStep,
         )
     elif ccxwriter.analysis_type == "buckling":
-        analysis_parameter = f"{ccxwriter.solver_obj.BucklingFactors}\n"
+        analysis_parameter = "{},{}".format(
+            ccxwriter.solver_obj.BucklingFactors,
+            ccxwriter.solver_obj.BucklingAccuracy,
+        )
 
     # write analysis type line, analysis parameter line
     f.write(analysis_type + "\n")