Density and viscosity as inputs instead of computed in computeMassFlux

Density and viscosity are given as input instead of calculated inside
computeMassFlux. This allow for modifying the properties prior to
calling computeMassFlux which avoids code duplication in the solvent
implementation.

opm/polymer/fullyimplicit/BlackoilPolymerModel.hpp

@@ -238,6 +238,8 @@ namespace Opm {
         computeMassFlux(const int               actph ,
                         const V&                transi,
                         const ADB&              kr    ,
+                        const ADB&              mu    ,
+                        const ADB&              rho   ,
                         const ADB&              p     ,
                         const SolutionState&    state );