[material][idealgas] change calculation of density, use molar density

Co-authored-by: Simon Scholz <simon.scholz@iws.uni-stuttgart.de>
Co-authored-by: Beatrix Becker <beatrix.becker@iws.uni-stuttgart.de>

dumux/material/idealgas.hh

@@ -50,7 +50,7 @@ public:
     static constexpr Scalar density(Scalar avgMolarMass,
                                     Scalar temperature,
                                     Scalar pressure)
-    { return pressure*avgMolarMass/(R*temperature); }
+    { return molarDensity(temperature,pressure)*avgMolarMass;}
 
     /*!
      * \brief The pressure of the gas in \f$\mathrm{[Pa]}\f$, depending on