[properties][slides] Use concrete Dumux example for Motivation

slides/properties.md

@@ -76,7 +76,6 @@ using GOF0 = Dune::GridOperator<
     CF, CF
 >;
 
-DGGO2 dggo2(gfs, cd, gfs, cf, lop, mbe);
 ```
 
 ## Traits classes
@@ -84,7 +83,7 @@ DGGO2 dggo2(gfs, cd, gfs, cf, lop, mbe);
 A usual way to group template parameters
 
 ```cpp
-template<class PV, class FSY, class FST, class SSY, class SST, class PT, class MT, class SR>
+template<class PV, class FSY, class FST, class SSY,...>
 struct TwoPVolumeVariablesTraits
 {
     using PrimaryVariables = PV;
@@ -97,31 +96,42 @@ struct TwoPVolumeVariablesTraits
     using SaturationReconstruction = SR;
 };
 ```
-## Traits classes
 
-Making it usable using a single template parameter
-```c++
-template<class TypeTag>
-struct VolumeVariables<TypeTag, TTag::TwoP>
+## Type traits
+
+Why do we need the type?
+
+```cpp
+// Type trait template declaration
+template<typename T> struct ValueType;
+
+// Specialization for vectors of T
+template<typename T, typename Allocator>
+struct ValueType<std::vector<T, Allocator>> { using type = T; };
+
+// Specialization for Dune::FieldVector
+template<typename T, int size>
+struct ValueType<Dune::FieldVector<T, size>> { using type = T; };
+```
+
+## Type traits
+
+```cpp
+
+template <class Traits>
+class TwoPVolumeVariables
+
 {
-private:
-    using PV = GetPropType<TypeTag, Properties::PrimaryVariables>;
-    using FSY = GetPropType<TypeTag, Properties::FluidSystem>;
-    using FST = GetPropType<TypeTag, Properties::FluidState>;
-    using SSY = GetPropType<TypeTag, Properties::SolidSystem>;
-    using SST = GetPropType<TypeTag, Properties::SolidState>;
-    using MT = GetPropType<TypeTag, Properties::ModelTraits>;
-    using PT = typename GetPropType<TypeTag, Properties::SpatialParams>::PermeabilityType;
-    using DM = typename GetPropType<TypeTag, Properties::GridGeometry>::DiscretizationMethod;
-    static constexpr bool enableIS = getPropValue<TypeTag, Properties::EnableBoxInterfaceSolver>();
-    // class used for scv-wise reconstruction of nonwetting phase saturations
-    using SR = TwoPScvSaturationReconstruction<DM, enableIS>;
-
-    using Traits = TwoPVolumeVariablesTraits<PV, FSY, FST, SSY, SST, PT, MT, SR>;
-public:
-    using type = TwoPVolumeVariables<Traits>;
-};
+   ...
+using FluidSystem = typename Traits::FluidSystem
+   ...
+   
+```
 
+Usage: these VolumeVariables will work for various FluidSystems:
+
+```cpp
+Scalar mu = FluidSystem::viscosity(fluidState, paramCache, phaseIdx);
 ```
 
 ## Inheriting from traits classes
@@ -143,24 +153,6 @@ struct MyDoubleTraits : public MyBaseTraits
 // this is a vector of ints!
 typename MyDoubleTraits::Vector v{1.14142, 1.73205};
 ```
-
-## Type traits
-
-Based on template specialization
-
-```cpp
-// Type trait template declaration
-template<typename T> struct ValueType;
-
-// Specialization for vectors of T
-template<typename T, typename Allocator>
-struct ValueType<std::vector<T, Allocator>> { using type = T; };
-
-// Specialization for Dune::FieldVector
-template<typename T, int size>
-struct ValueType<Dune::FieldVector<T, size>> { using type = T; };
-```
-
 # The DuMuX Property System
 
 ## Property System Design