[Kinetics] Fix third body concentrations for non-ideal gases

From the discussion on Cantera#967, the calcuation of [M] should be based on
the physical concentrations of the 3rd body colliders, not their
"activity concentrations".

include/cantera/kinetics/BulkKinetics.h

@@ -62,7 +62,12 @@ class BulkKinetics : public Kinetics
     //! valued stoichiometries.
     vector_fp m_dn;
 
-    vector_fp m_conc;
+    //! Activity concentrations, as calculated by
+    //! ThermoPhase::getActivityConcentrations
+    vector_fp m_act_conc;
+
+    //! Physical concentrations, as calculated by ThermoPhase::getConcentrations
+    vector_fp m_phys_conc;
     vector_fp m_grt;
 
     bool m_ROP_ok;

include/cantera/kinetics/GasKinetics.h

@@ -101,6 +101,7 @@ class GasKinetics : public BulkKinetics
     vector_fp falloff_work;
     vector_fp concm_3b_values;
     vector_fp concm_falloff_values;
+
     //!@}
 
     void processFalloffReactions();

src/kinetics/BulkKinetics.cpp

@@ -178,7 +178,8 @@ void BulkKinetics::modifyElementaryReaction(size_t i, ElementaryReaction& rNew)
 void BulkKinetics::resizeSpecies()
 {
     Kinetics::resizeSpecies();
-    m_conc.resize(m_kk);
+    m_act_conc.resize(m_kk);
+    m_phys_conc.resize(m_kk);
     m_grt.resize(m_kk);
 }
 

src/kinetics/GasKinetics.cpp

@@ -68,17 +68,18 @@ void GasKinetics::update_rates_T()
 
 void GasKinetics::update_rates_C()
 {
-    thermo().getActivityConcentrations(m_conc.data());
+    thermo().getActivityConcentrations(m_act_conc.data());
+    thermo().getConcentrations(m_phys_conc.data());
     doublereal ctot = thermo().molarDensity();
 
     // 3-body reactions
     if (!concm_3b_values.empty()) {
-        m_3b_concm.update(m_conc, ctot, concm_3b_values.data());
+        m_3b_concm.update(m_phys_conc, ctot, concm_3b_values.data());
     }
 
     // Falloff reactions
     if (!concm_falloff_values.empty()) {
-        m_falloff_concm.update(m_conc, ctot, concm_falloff_values.data());
+        m_falloff_concm.update(m_phys_conc, ctot, concm_falloff_values.data());
     }
 
     // P-log reactions
@@ -187,10 +188,10 @@ void GasKinetics::updateROP()
     }
 
     // multiply ropf by concentration products
-    m_reactantStoich.multiply(m_conc.data(), m_ropf.data());
+    m_reactantStoich.multiply(m_act_conc.data(), m_ropf.data());
 
     // for reversible reactions, multiply ropr by concentration products
-    m_revProductStoich.multiply(m_conc.data(), m_ropr.data());
+    m_revProductStoich.multiply(m_act_conc.data(), m_ropr.data());
 
     for (size_t j = 0; j != nReactions(); ++j) {
         m_ropnet[j] = m_ropf[j] - m_ropr[j];