[FEM] Elmer: add support to turn off solvers

- with this property the linear solver is turned off. As consequence one can this way also disable the execution of an equation without the need to delete the equation object

- also add missing preconditioners

src/Mod/Fem/femsolver/elmer/equations/linear.py

@@ -46,15 +46,17 @@ LINEAR_ITERATIVE = [
     "TFQMR",
     "GMRES",
     "GCR",
+    "Idrs"
 ]
 LINEAR_PRECONDITIONING = [
     "None",
     "Diagonal",
+    "ILUT",
     "ILU0",
     "ILU1",
     "ILU2",
     "ILU3",
-    "ILU4",
+    "ILU4"
 ]
 
 
@@ -118,6 +120,16 @@ class Proxy(equation.Proxy):
             ""
         )
         obj.LinearIterations = 500
+        obj.addProperty(
+            "App::PropertyBool",
+            "LinearSystemSolverDisabled",
+            "Linear System",
+            (
+                "Disable the linear system.\n"
+                "Only use for special cases\n"
+                "and consult the Elmer docs."
+            )
+        )
         obj.addProperty(
             "App::PropertyFloat",
             "SteadyStateTolerance",

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

@@ -1489,6 +1489,17 @@ class Writer(object):
             equation.setExpression("SteadyStateTolerance", str(equation.SteadyStateTolerance))
         if equation.BiCGstablDegree == 0:
             equation.BiCGstablDegree = 2
+        if not hasattr(equation, "LinearSystemSolverDisabled"):
+            equation.addProperty(
+                "App::PropertyBool",
+                "LinearSystemSolverDisabled",
+                "Linear System",
+                (
+                    "Disable the linear system.\n"
+                    "Only use for special cases\n"
+                    "and consult the Elmer docs."
+                )
+            )
 
     def _createLinearSolver(self, equation):
         # first check if we have to update
@@ -1497,6 +1508,7 @@ class Writer(object):
         s = sifio.createSection(sifio.SOLVER)
         s.priority = equation.Priority
         s["Linear System Solver"] = equation.LinearSolverType
+        s["Linear System Solver Disabled"] = equation.LinearSystemSolverDisabled
         if equation.LinearSolverType == "Direct":
             s["Linear System Direct Method"] = \
                 equation.LinearDirectMethod