[1pmodel] Function calls with phaseIdx

Added phaseIdx to VolVars functions such that
it is consistent with other models.

Reviewed by Bernd

dumux/implicit/1p/1pvolumevariables.hh

@@ -130,22 +130,22 @@ public:
      * \brief Return the effective pressure \f$\mathrm{[Pa]}\f$ of a given phase within
      *        the control volume.
      */
-    Scalar pressure() const
-    { return fluidState_.pressure(/*phaseIdx=*/0); }
+    Scalar pressure(int phaseIdx = 0) const
+    { return fluidState_.pressure(phaseIdx); }
 
     /*!
      * \brief Return the mass density \f$\mathrm{[kg/m^3]}\f$ of a given phase within the
      *        control volume.
      */
-    Scalar density() const
-    { return fluidState_.density(/*phaseIdx=*/0); }
+    Scalar density(int phaseIdx = 0) const
+    { return fluidState_.density(phaseIdx); }
 
     /*!
      * \brief Return the dynamic viscosity \f$\mathrm{[Pa s]}\f$ of the fluid within the
      *        control volume.
      */
-    Scalar viscosity() const
-    { return fluidState_.viscosity(/*phaseIdx=*/0); }
+    Scalar viscosity(int phaseIdx = 0) const
+    { return fluidState_.viscosity(phaseIdx); }
 
     /*!
      * \brief Returns the mobility \f$\mathrm{[1/(Pa s)]}\f$.