[MPL] Add hasPhase to medium and fluidPhase().

For some processes, like HM, there is no difference between
the gas and liquid phases, a fluid phase is sufficient.

MaterialLib/MPL/Medium.cpp

@@ -40,6 +40,12 @@ Phase const& Medium::phase(std::string const& name) const
             return phase->name == name;
         },
         "Could not find phase name '" + name + "'.");
+
+bool Medium::hasPhase(std::string const& phase_name) const
+{
+    return std::any_of(
+        begin(phases_), end(phases_),
+        [&phase_name](auto const& phase) { return phase->name == phase_name; });
 }
 
 Property const& Medium::property(PropertyType const& p) const
@@ -67,4 +73,19 @@ std::string Medium::description()
 {
     return "medium";
 }
+
+Phase const& fluidPhase(Medium const& medium)
+{
+    if (medium.hasPhase("Gas"))
+    {
+        return medium.phase("Gas");
+    }
+    if (medium.hasPhase("AqueousLiquid"))
+    {
+        return medium.phase("AqueousLiquid");
+    }
+    OGS_FATAL(
+        "Neither Gas nor AqueousLiquid phase is available for the medium, but "
+        "a fluid phase was requested.");
+}
 }  // namespace MaterialPropertyLib

MaterialLib/MPL/Medium.h

@@ -38,6 +38,10 @@ public:
     Phase const& phase(std::size_t index) const;
     /// A get-function for a particular phase by phase name.
     Phase const& phase(std::string const& phase_name) const;
+
+    /// A query for a named phase.
+    bool hasPhase(std::string const& phase_name) const;
+
     /// A get-function for a property. The argument refers to the name of the
     /// property.
     Property const& property(PropertyType const& p) const;
@@ -107,4 +111,7 @@ void checkRequiredProperties(Medium const& medium,
     }
 }
 
+/// Returns a gas or aqueous liquid phase of the given medium.
+Phase const& fluidPhase(Medium const& medium);
+
 }  // namespace MaterialPropertyLib