[Thermo] Implement missing SurfPhase functions

Adding the entropy_mole, cp_mole, and cv_mole functions fixes the silent failure
of the report() function for surface phases.

include/cantera/thermo/SurfPhase.h

@@ -228,6 +228,18 @@ public:
      */
     virtual doublereal intEnergy_mole() const;
 
+    //! Return the Molar Entropy. Units: J/kmol-K
+    /**
+     * \f[
+     *  \hat s(T,P) = \sum_k X_k (\hat s^0_k(T) - R \log(\theta_k))
+     * \f]
+     */
+    virtual doublereal entropy_mole() const;
+
+    virtual doublereal cp_mole() const;
+
+    virtual doublereal cv_mole() const;
+
     //! Get the species chemical potentials. Units: J/kmol.
     /*!
      * This function returns a vector of chemical potentials of the

src/thermo/SurfPhase.cpp

@@ -113,6 +113,28 @@ doublereal SurfPhase::intEnergy_mole() const
     return enthalpy_mole();
 }
 
+doublereal SurfPhase::entropy_mole() const
+{
+    _updateThermo();
+    doublereal s = 0.0;
+    for (size_t k = 0; k < m_kk; k++) {
+        s += moleFraction(k) * (m_s0[k] -
+            GasConstant * log(std::max(concentration(k) * size(k)/m_n0, SmallNumber)));
+    }
+    return s;
+}
+
+doublereal SurfPhase::cp_mole() const
+{
+    _updateThermo();
+    return mean_X(&m_cp0[0]);
+}
+
+doublereal SurfPhase::cv_mole() const
+{
+    return cp_mole();
+}
+
 void SurfPhase::getPartialMolarEnthalpies(doublereal* hbar) const
 {
     getEnthalpy_RT(hbar);