More const for vdW

include/teqp/models/vdW.hpp

@@ -19,8 +19,8 @@ public:
     vdWEOS1(double a, double b) : a(a), b(b) {};
     
     /// Accessor functions
-    double get_a(){ return a; }
-    double get_b(){ return b; }
+    double get_a() const{ return a; }
+    double get_b() const{ return b; }
 
     const double Ru = 1.380649e-23 * 6.02214076e23; ///< Exact value, given by k_B*N_A