Return perm by reference from volvars

for all ported models (1p, 1p2c, 2p, 2p2c, 2pnc, richards)

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

@@ -171,7 +171,7 @@ public:
     /*!
      * \brief Returns the permeability within the control volume in \f$[m^2]\f$.
      */
-    PermeabilityType permeability() const
+    const PermeabilityType& permeability() const
     { return permeability_; }
 
     /*!

dumux/porousmediumflow/1p2c/implicit/volumevariables.hh

@@ -276,7 +276,7 @@ public:
     /*!
      * \brief Returns the permeability within the control volume in \f$[m^2]\f$.
      */
-    PermeabilityType permeability() const
+    const PermeabilityType& permeability() const
     { return permeability_; }
 
 protected:

dumux/porousmediumflow/2p/implicit/volumevariables.hh

@@ -239,7 +239,7 @@ public:
     /*!
      * \brief Returns the permeability within the control volume in \f$[m^2]\f$.
      */
-    PermeabilityType permeability() const
+    const PermeabilityType& permeability() const
     { return permeability_; }
 
 protected:

dumux/porousmediumflow/2p2c/implicit/volumevariables.hh

@@ -559,7 +559,7 @@ public:
     /*!
      * \brief Returns the average permeability within the control volume in \f$[m^2]\f$.
      */
-    PermeabilityType permeability() const
+    const PermeabilityType& permeability() const
     { return permeability_; }
 
     /*!

dumux/porousmediumflow/2pnc/implicit/volumevariables.hh

@@ -452,7 +452,7 @@ public:
     /*!
      * \brief Returns the permeability within the control volume.
      */
-    PermeabilityType permeability() const
+    const PermeabilityType& permeability() const
     { return permeability_; }
 
 

dumux/porousmediumflow/richards/implicit/volumevariables.hh

@@ -156,7 +156,7 @@ public:
     /*!
      * \brief Returns the permeability within the control volume in \f$[m^2]\f$.
      */
-    PermeabilityType permeability() const
+    const PermeabilityType& permeability() const
     { return permeability_; }
 
     /*!