ECL thermal gas PVT: add the heat of vaporization to the gas enthalpy

this does only matters for temperature changes due to dissolution and
evaporization of gas in oil or oil in gas. The comprehensive solution
would be to make the enthalpy of the phases composition dependent, but
ECL does not seem to support this when using the black-oil model.

opm/material/fluidsystems/blackoilpvt/GasPvtThermal.hpp

@@ -131,7 +131,14 @@ public:
 
                 std::vector<double> hSamples(temperatureColumn.size());
 
-                Scalar h = temperatureColumn[0]*cpGasColumn[0];
+                // the specific enthalpy of vaporization. since ECL does not seem to
+                // feature a proper way to specify this quantity, we use the value for
+                // methane. A proper model would also need to consider the enthalpy
+                // change due to dissolution, i.e. the enthalpies of the gas and oil
+                // phases should depend on the phase composition
+                const Scalar hVap = 480.6e3; // [J / kg]
+
+                Scalar h = temperatureColumn[0]*cpGasColumn[0] + hVap;
                 for (size_t i = 0;; ++i) {
                     hSamples[i] = h;