[Kinetics] added const version of Kinetics::reaction

include/cantera/kinetics/Kinetics.h

@@ -782,6 +782,8 @@ public:
      * Return the Reaction object for reaction *i*.
      */
     shared_ptr<Reaction> reaction(size_t i);
+    
+    shared_ptr<const Reaction> reaction(size_t i) const;
 
     //! Determine behavior when adding a new reaction that contains species not
     //! defined in any of the phases associated with this kinetics manager. If

src/kinetics/Kinetics.cpp

@@ -649,5 +649,11 @@ shared_ptr<Reaction> Kinetics::reaction(size_t i)
     checkReactionIndex(i);
     return m_reactions[i];
 }
+    
+shared_ptr<const Reaction> Kinetics::reaction(size_t i) const
+{
+    checkReactionIndex(i);
+    return m_reactions[i];
+}
 
 }