[tpfa][fourier] use const variables where possible

dumux/discretization/cellcentered/tpfa/fourierslaw.hh

@@ -144,8 +144,12 @@ public:
         const auto& insideScv = fvGeometry.scv(insideScvIdx);
         const auto& insideVolVars = elemVolVars[insideScvIdx];
 
-        auto insideLambda = ThermalConductivityModel::effectiveThermalConductivity(insideVolVars, problem.spatialParams(), element, fvGeometry, insideScv);
-        Scalar ti = computeTpfaTransmissibility(scvf, insideScv, insideLambda, insideVolVars.extrusionFactor());
+        const auto insideLambda = ThermalConductivityModel::effectiveThermalConductivity(insideVolVars,
+                                                                                         problem.spatialParams(),
+                                                                                         element,
+                                                                                         fvGeometry,
+                                                                                         insideScv);
+        const Scalar ti = computeTpfaTransmissibility(scvf, insideScv, insideLambda, insideVolVars.extrusionFactor());
 
         // for the boundary (dirichlet) or at branching points we only need ti
         if (scvf.boundary() || scvf.numOutsideScvs() > 1)
@@ -160,11 +164,11 @@ public:
             const auto& outsideVolVars = elemVolVars[outsideScvIdx];
             const auto outsideElement = fvGeometry.fvGridGeometry().element(outsideScvIdx);
 
-            auto outsideLambda = ThermalConductivityModel::effectiveThermalConductivity(outsideVolVars,
-                                                                                        problem.spatialParams(),
-                                                                                        outsideElement,
-                                                                                        fvGeometry,
-                                                                                        outsideScv);
+            const auto outsideLambda = ThermalConductivityModel::effectiveThermalConductivity(outsideVolVars,
+                                                                                              problem.spatialParams(),
+                                                                                              outsideElement,
+                                                                                              fvGeometry,
+                                                                                              outsideScv);
             Scalar tj;
             if (dim == dimWorld)
                 // assume the normal vector from outside is anti parallel so we save flipping a vector