[material] Re-Include partial pressure in immiscible fluidstate

The function returning the partial pressure was accidentally removed
from the immiscible fluidstate. This caused an compiler error, e.g., when
the water-air fluidsystem is used in 2p models.

Reviewed by Thomas

dumux/material/fluidstates/immisciblefluidstate.hh

@@ -128,6 +128,21 @@ public:
             return std::numeric_limits<Scalar>::infinity();
     }
 
+    /*!
+     * \brief The partial pressure of a component in a phase \f$\mathrm{[Pa]}\f$
+     *
+     * To avoid numerical issues with code that assumes miscibility,
+     * we return a partial pressure of 0 for components which do not mix with
+     * the specified phase. Actually it is undefined.
+     */
+    Scalar partialPressure(int phaseIdx, int compIdx) const
+    {
+        if (phaseIdx == compIdx)
+            return pressure(phaseIdx);
+        else
+            return 0;
+    }
+
     /*!
      * \brief The molar volume of a fluid phase \f$\mathrm{[m^3/mol]}\f$
      *