[implicit][2p] Add function which returns the viscosity of the phases

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

@@ -217,6 +217,15 @@ public:
     Scalar temperature() const
     { return fluidState_.temperature(/*phaseIdx=*/0); }
 
+    /*!
+     * \brief Returns the dynamic viscosity of the fluid within the
+     *        control volume in \f$\mathrm{[Pa s]}\f$.
+     *
+     * \param phaseIdx The phase index
+     */
+    Scalar viscosity(int phaseIdx) const
+    { return fluidState_.viscosity(phaseIdx); }
+
     /*!
      * \brief Returns the effective mobility of a given phase within
      *        the control volume in \f$[s*m/kg]\f$.