[cleanup] Use convenience function for pressure and density

Implement a pressure()  and density() function in the volvars and use it
instead of volvars.fluidstate().x(phaseIdx).

dumux/porousmediumflow/mpnc/implicit/mass/localresidualkinetic.hh

@@ -196,7 +196,7 @@ public:
 
         const Scalar characteristicLength   = volVars.characteristicLength()  ;
         const Scalar temperature            = volVars.fluidState().temperature(wPhaseIdx);
-        const Scalar pn                     = volVars.fluidState().pressure(nPhaseIdx);
+        const Scalar pn                     = volVars.pressure(nPhaseIdx);
         const Scalar henry                  = FluidSystem::henry(temperature) ;
         const Scalar gradNinWApprox  = ( mu_wPhaseNComp - mu_nPhaseNCompEquil) / characteristicLength;    // very 2p2c // 1. / henry *
         const Scalar gradWinNApprox  = ( mu_nPhaseWComp - mu_wPhaseWCompEquil) / characteristicLength;    // very 2p2c // 1. / pn *

dumux/porousmediumflow/mpnc/implicit/mass/vtkwriterkinetic.hh

@@ -127,7 +127,7 @@ public:
 //            }
 
             if (deltaPOutput_) {
-                deltaP_[vIdxGlobal] = volVars.fluidState().pressure(nPhaseIdx) - 100000.;
+                deltaP_[vIdxGlobal] = volVars.pressure(nPhaseIdx) - 100000.;
             }
         }
     }

dumux/porousmediumflow/mpnc/implicit/volumevariables.hh

@@ -308,6 +308,19 @@ public:
     Scalar molarDensity(const int phaseIdx) const
     { return fluidState_.molarDensity(phaseIdx);}
 
+    /*!
+     * \brief Return the effective pressure \f$\mathrm{[Pa]}\f$ of a given phase within
+     *        the control volume.
+     */
+    Scalar pressure(const int phaseIdx) const
+    { return fluidState_.pressure(phaseIdx); }
+
+    /*!
+     * \brief Return density \f$\mathrm{[kg/m^3]}\f$ the of the fluid phase.
+     */
+    Scalar density(const int phaseIdx) const
+    { return fluidState_.density(phaseIdx); }
+
     /*!
      * \brief Returns the effective mobility of a given phase within
      *        the control volume.

dumux/porousmediumflow/mpnc/implicit/vtkwritercommon.hh

@@ -152,8 +152,8 @@ public:
 
             for (int phaseIdx = 0; phaseIdx < numPhases; ++phaseIdx) {
                 if (saturationOutput_) saturation_[phaseIdx][dofIdxGlobal] = volVars.saturation(phaseIdx);
-                if (pressureOutput_) pressure_[phaseIdx][dofIdxGlobal] = volVars.fluidState().pressure(phaseIdx);
-                if (densityOutput_) density_[phaseIdx][dofIdxGlobal] = volVars.fluidState().density(phaseIdx);
+                if (pressureOutput_) pressure_[phaseIdx][dofIdxGlobal] = volVars.pressure(phaseIdx);
+                if (densityOutput_) density_[phaseIdx][dofIdxGlobal] = volVars.density(phaseIdx);
                 if (mobilityOutput_) mobility_[phaseIdx][dofIdxGlobal] = volVars.mobility(phaseIdx);
                 if (averageMolarMassOutput_) averageMolarMass_[phaseIdx][dofIdxGlobal] = volVars.fluidState().averageMolarMass(phaseIdx);
                 for (int compIdx = 0; compIdx < numComponents; ++compIdx) {

test/porousmediumflow/mpnc/implicit/combustionproblem1c.hh

@@ -478,8 +478,8 @@ public:
 
         FluidState fluidState;
 
-        const Scalar pn = elemVolVars[scvIdx].fluidState().pressure(nPhaseIdx);
-        const Scalar pw = elemVolVars[scvIdx].fluidState().pressure(wPhaseIdx);
+        const Scalar pn = elemVolVars[scvIdx].pressure(nPhaseIdx);
+        const Scalar pw = elemVolVars[scvIdx].pressure(wPhaseIdx);
 
         const Scalar Tn = elemVolVars[scvIdx].fluidState().temperature(nPhaseIdx);
         const Scalar Tw = elemVolVars[scvIdx].fluidState().temperature(wPhaseIdx);