[volvars] introduce convenience function to extract privars from element solution

dumux/discretization/volumevariables.hh

@@ -82,10 +82,17 @@ public:
                 const Element &element,
                 const SubControlVolume &scv)
     {
-        priVars_ = isBox ? elemSol[scv.index()] : elemSol[0];
+        priVars_ = extractDofPriVars(elemSol, scv);
         extrusionFactor_ = problem.extrusionFactor(element, scv, elemSol);
     }
 
+    /*!
+     * \brief Returns the primary variables at the dof associated with a given scv.
+     */
+    static const PrimaryVariables& extractDofPriVars(const ElementSolutionVector& elemSol,
+                                                     const SubControlVolume& scv)
+    { return isBox ? elemSol[scv.index()] : elemSol[0]; }
+
     /*!
      * \brief Return the vector of primary variables
      */

dumux/porousmediumflow/1p/implicit/volumevariables.hh

@@ -91,7 +91,7 @@ public:
         fluidState.setTemperature(t);
         fluidState.setSaturation(/*phaseIdx=*/0, 1.);
 
-        const auto& priVars = isBox ? elemSol[scv.index()] : elemSol[0];
+        const auto& priVars = ParentType::extractDofPriVars(elemSol, scv);
         fluidState.setPressure(/*phaseIdx=*/0, priVars[Indices::pressureIdx]);
 
         // saturation in a single phase is always 1 and thus redundant