[richards] Make fluid system check reusable and check in several places

dumux/porousmediumflow/richards/localresidual.hh

@@ -79,6 +79,9 @@ class RichardsLocalResidual : public GetPropType<TypeTag, Properties::BaseLocalR
     // first index for the mass balance
     enum { conti0EqIdx = Indices::conti0EqIdx };
 
+    // checks if the fluid system uses the Richards model index convention
+    static constexpr auto fsCheck = GetPropType<TypeTag, Properties::ModelTraits>::checkFluidSystem(FluidSystem{});
+
     // phase & component indices
     static constexpr auto liquidPhaseIdx = FluidSystem::phase0Idx;
     static constexpr auto gasPhaseIdx = FluidSystem::phase1Idx;

dumux/porousmediumflow/richards/model.hh

@@ -133,6 +133,27 @@ struct RichardsModelTraits
     static constexpr bool enableAdvection() { return true; }
     static constexpr bool enableMolecularDiffusion() { return enableDiff; }
     static constexpr bool enableEnergyBalance() { return false; }
+
+    //! The Richards model has some assumptions on the fluid systems
+    //! that can be verified with this trait
+    template<class FluidSystem>
+    static constexpr bool fluidSystemIsCompatible()
+    {
+        return !FluidSystem::isGas(FluidSystem::phase0Idx)
+            && FluidSystem::isGas(FluidSystem::phase1Idx);
+    }
+
+    //! The Richards model has some assumptions on the fluid systems
+    //! that can be verified with this trait
+    template<class FluidSystem>
+    static constexpr auto checkFluidSystem(const FluidSystem& fs)
+    {
+        struct FluidSystemCheck {
+            static_assert(fluidSystemIsCompatible<FluidSystem>(),
+                "Richards model currently assumes the first phase to be liquid and the second phase to be gaseous.");
+        };
+        return FluidSystemCheck{};
+    }
 };
 
 /*!

dumux/porousmediumflow/richards/volumevariables.hh

@@ -75,6 +75,9 @@ class RichardsVolumeVariables
 
     using EffDiffModel = typename Traits::EffectiveDiffusivityModel;
 
+    // checks if the fluid system uses the Richards model index convention
+    static constexpr auto fsCheck = ModelTraits::checkFluidSystem(typename Traits::FluidSystem{});
+
 public:
     //! Export type of the fluid system
     using FluidSystem = typename Traits::FluidSystem;
@@ -93,9 +96,6 @@ public:
     static constexpr auto liquidPhaseIdx = Traits::FluidSystem::phase0Idx;
     static constexpr auto gasPhaseIdx = Traits::FluidSystem::phase1Idx;
 
-    static_assert(!Traits::FluidSystem::isGas(liquidPhaseIdx), "Richards model assumes first phase to be liquid");
-    static_assert(Traits::FluidSystem::isGas(gasPhaseIdx), "Richards model assumes second phase to be gaseous");
-
     /*!
      * \brief Updates all quantities for a given control volume.
      *