[brineco2] add missing gasIsIdeal() function

dumux/material/fluidsystems/brineco2.hh

@@ -218,6 +218,21 @@ public:
         return phaseIdx == gasPhaseIdx;
     }
 
+    /*!
+     * \brief Returns true if and only if a fluid phase is assumed to
+     *        be an ideal gas.
+     * \param phaseIdx The index of the fluid phase to consider
+     */
+    static constexpr bool isIdealGas(int phaseIdx)
+    {
+        assert(0 <= phaseIdx && phaseIdx < numPhases);
+        // let the fluids decide
+        if (phaseIdx == gasPhaseIdx)
+            return useConstantSalinity ? (ConstantSalinityBrine::gasIsIdeal() && CO2::gasIsIdeal())
+                                       : (H2O::gasIsIdeal() && CO2::gasIsIdeal());
+        return false; // not a gas
+    }
+
     /*!
      * \brief Returns true if and only if a fluid phase is assumed to
      *        be an ideal mixture.