[fluidsystems][h2o2n2liquidphase] Fix deprecation

* instead the h2o2n2 fluidsystem can be called and the phase index can be set in the proeprty system

dumux/material/fluidsystems/h2on2liquidphasefluidsystem.hh

@@ -5,4 +5,44 @@
 
 #include <dumux/material/fluidsystems/h2on2.hh>
 
+namespace Dumux
+{
+namespace FluidSystems
+{
+
+/*!
+ * \ingroup Fluidsystems
+ *
+ * \brief A one-phase (water-phase) fluid system with water and nitrogen as components.
+ *
+ * This FluidSystem can be used without the PropertySystem that is applied in Dumux,
+ * as all Parameters are defined via template parameters. Hence it is in an
+ * additional namespace Dumux::FluidSystems::.
+ * An adapter class using Dumux::FluidSystem<TypeTag> is also provided
+ * at the end of this file.
+ */
+template <class Scalar, bool useComplexRelations = true>
+DUNE_DEPRECATED_MSG("Class Dumux::FluidSystems::H2ON2LiquidPhase is deprecated. Use Dumux::FluidSystems::H2ON2 instead.")
+class H2ON2LiquidPhase
+    : public H2ON2<Scalar, useComplexRelations>
+{};
+
+} // end namespace FluidSystems
+
+#ifdef DUMUX_PROPERTIES_HH
+/*!
+ * \brief A one-phase fluid system with water and nitrogen as components.
+ *
+ * This is an adapter to use Dumux::H2ON2LiquidPhaseFluidSystem<TypeTag>, as is
+ * done with most other classes in Dumux.
+ */
+template<class TypeTag>
+DUNE_DEPRECATED_MSG("Class Dumux::H2ON2LiquidPhaseFluidSystem is deprecated. Use Dumux::H2ON2FluidSystem instead.")
+class H2ON2LiquidPhaseFluidSystem
+: public H2ON2FluidSystem<TypeTag>
+{};
+#endif
+
+} // end namespace
+
 #endif